1. 1. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 1 ACKNOWLEDGEMENT First and foremost we wish to express our gratitude towards Dr. Ravishankar R., Head of Department of Chemical Engineering, for all the help extended to us at every stage of the project. We are indebted to Mr. Sunil H., our guide and Assistant Professor in the Department of Chemical Engineering, for his valuable guidance, encouragement and suggestions through the course of the project. Without his guidance this project would not have been completed. We are also grateful to Mrs. Vidhya Karthikeyan, SCI - Engr SD and Mr R Sundara Rajan , Manger (STF), LPSC, ISRO for their valuable guidance and such a wonderful opportunity to be a part of such an esteemed institution. Finally, we would like to thanks our parents for their unending support to us in all endeavours that we pursue.
2. 2. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 2 TABLE OF CONTENTS I. List of Figures Figure No. Description 2.1 Electrochemical Attack 2.2 Cost of corrosion in the US 2.3 Change in thickness of metal 2.4 Effect of voltage and temperature on unit barrier thickness 2.5 Relationship between voltage, current density and temperature 2.6 Relationship between concentration of electrolyte and unit barrier layer thickness 2.7 Effect of bath temperature on porosity 2.8 Relationship between bath voltage and current density to treatment time 2.9 Effect on anodizing time on film growth and dimension of work-piece 2.10 Coating ratios for various alloys 2.11 Automated ultrasonic cleaning system 2.12 Conversion of existing normal tank to ultrasonic tank 3.1 Schematic diagram of anodization tank 3.2 Dyed specimen
3. 3. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 3 3.3 Schematic diagram for vapour degreasing 3.4 Cavitation 3.5 Schematic diagram for ultrasonic solvent cleaner 4.1 Thickness VS time 4.2 Temperature VS gain in weight
4. 4. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 4 II. List of Tables Table No. Description 2.1 Galvanic series of metals 2.2 Specifications of anodization in aerospace series 2.3 Oxide films produced by various treatments 2.4 Coating of oxide films 2.5 Effect of operating conditions on properties of coating bath 2.6 Anodic oxide coating composition 2.7 Initial rating of CASS 2.8 Selection of cleaning agents for ultrasonic solvent cleaning 3.1 Process sheet for pickling- passivation 4.1 Relationship of coating thickness with anodizing time 4.2 Relationship between weight gain and temperature 4.3 Initial rating of CASS 4.4 Intensity of Stain
5. 5. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 5 III. Abstract 09 Chapter 1 Introduction Introduction 11 Chapter 2 Literature Survey 2.1 Corrosion 13 2.2 Mechanism of Corrosion 15 2.2.1 Electrochemical Attack 16 2.2.2 Basic Process 19 2.3 Chemistry of Corrosion 19 2.4 Factors that Control Corrosion 20 2.5 Forms of Corrosion 22 2.5.1 Surface Corrosion 22 2.5.2 Dissimilar metal Corrosion 22 2.5.3 Inter Granular Corrosion 23 2.5.4 Stress Corrosion 23 2.5.5 Fretting Corrosion 24 2.6 Consequences of Corrosion 24 2.7 Anodization 26 2.7.1 History 27 2.7.2 Anodized Aluminium 27 2.7.3 Specifications 29 2.7.4 Mechanism of Anodization Process 30 2.7.5 Barrier Layer 32 2.7.5.1 Thickness 32 2.7.5.2 Effect of Operating Conditions On Barrier Layer 34 2.7.6 Porous Layer 37 2.7.6.1 Porosity 37 2.7.6.2 Mechanism of Porous Film Growth 39 2.7.7 Coating Ratio 40 2.7.8 Anodic Oxide Coating Composition 44 2.7.9 Comparison of AC & DC Anodization 46 2.8 Properties & Tests of Anodic Oxide Coating 47 2.8.1 Apparent Density 47
6. 6. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 6 2.8.2 Coating Thickness 48 2.8.3 Porosity 52 2.8.4 Adhesion 54 2.8.5 Sealing Efficiency 55 2.8.6 Corrosion 56 2.9 Types of Anodization 59 2.9.1 Chromic Acid Anodizing (type I) 59 2.9.2 Sulphuric Acid Anodizing (type II & III) 59 2.9.3 Organic Acid Anodizing 60 2.10 Dyes and Colours 60 2.11 Sealing 61 2.12 Mechanical Considerations 62 2.13 Laboratory Testing 62 2.14 Environmental Impacts 62 2.15 Ultrasonic Solvent Cleaning 64 2.15.1 System Design 67 2.16 Pickling-Passivation 71 2.16.1 Pickling 71 2.16.2 Passivation 71 2.16.3 Test for Detemining Effectiveness of Passivation 73 2.17 Chemical Cleaning 75 2.17.1 Types of Chemical Cleaners 75 2.17.2 Common Cleaning Agents Used 76 2.18 Vapour Degreasing 77 2.19 Objective 78 Chapter 3 Materials and Methods 3.1 Materials 80 3.2 Pre-treatment of Aluminium for Anodization 80 3.2.1 Mechanical Cleaning 81 3.2.2 Ultrasonic Cleaning 81 3.2.3 Acetone Rinsing 82
7. 7. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 7 3.2.4 Drying 82 3.2.5 Alkali Cleaning 82 3.2.6 Rinsing in Water 82 3.2.7 Acid Cleaning 82 3.2.8 Rinsing with DM Water 83 3.2.9 Process Flow sheet for Cleaning Cycle 84 3.3 Anodization Process 85 3.2.1 Construction of Anodization Tank 85 3.2.2 Process Description 86 3.4 Post-treatment of Anodized Aluminium 87 3.4.1 Dyeing 87 3.4.2 Sealing 88 3.4.3 Process Flowchart 90 3.5 Pickling-passivation Process Sheet 91 3.5.1 Process flowchart for pickling-passivation 93 3.5.2 Solvent Cleaning 94 3.5.3 Alkali Cleaning 94 3.5.4 Rinsing in Water 94 3.5.5 Pickling 94 3.5.6 Rinsing in Water 94 3.5.7 Passivation 94 3.5.8 Rinsing in Water 95 3.5.9 Rinsing in DM Water 95 3.5.10 Drying 95 3.6 Flowchart for chemical cleaning of satellite tankages 97 3.7 Vapour Degreasing Technical Specifications 97 3.7.1 Vapour Degreasing System Description 98 3.8 Ultrasonic Solvent Cleaning System Description 100 3.8.1 Ultrasonic solvent Cleaning Technical Specification 101
8. 8. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 8 Chapter 4 Results and Discussion 4.1 Test for Thickness 104 4.2 Test of gain in Weight with Temperature 105 4.3 Test for Porosity 106 4.4 Corrosion Test (CASS test) 107 4.5 Test for Sealing 108 Chapter 5 Conclusion Conclusion 112 Chapter 6 References Bibliography 114
9. 9. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 9 ABSTRACT Anodization is the common designation for Anodic oxidation of the certain metals to form stable oxide film on their surface. But prior to anodization or other coating on metal surfaces, certain other surface treatments are employed there include pickling-passivation, vapour degreasing, ultrasonic solvent cleaning andor chemical cleaning. All these methods in some way or the other render corrosion and abrasion resistance to the metal surfaces. Aerospace industries employed such surface treatment methods mainly for providing resistance to the metal components against corrosion. The metals used predominantly include Aluminium, Magnesium, and Stainless Steel of various grades and Titanium. The liquid propulsion system center, ISRO, specializes in treating surfaces of components parts of satellite launch vehicles. Being offered to carry out project work at this highly prestigious institution is a matter of great pride. The project carried out was challenging as high specification and accuracy had to be maintained while performing the experimental work. Tests were carried out to realize which treatment methods could be employed to which specific metal for using aerospace industry and whether requirements like thickness, hardness, porosity, corrosion resistance etc, were up to the required specifications.
10. 10. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 10 Chapter 1 Introduction
11. 11. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 11 INTRODUCTION Most metals exist in nature in combined form as their oxides, carbonates, hydroxyl, chlorides, and silicates. During extraction these are reduced to their metallic state from their ores and extraction considerable amount of energy required. Consequently, isolated pure metals can be regarded as I excited state than corresponding ores and they have natural tendency to revert back to the combined state. Hence when metals are put into use in various forms they are exposed to the environment such as dry gases, liquids etc. thus destruction of metals start at the surface. This type of metal destruction may be due to direct chemical corrosion by the environment or by electrochemical attack. Any process of deteoriation of metal, through an unwanted chemical or electrochemical attack, starting from its surface is termed as corrosion. The process of corrosion is slow and occurs only at surface of metals, but losses incurred due to corrosion are enormous. In general, the life and strength of structure is reduced very much due to corrosion is 1/5th of the total world production. It is very difficult to assess the exact losses incurred due to corrosion. Various methods have been developed to protect metals and to prevent corrosion. But even today there is no method used that can assure 100% protection. The most common methods employed are painting, electroplating, anodization, galvanizing etc. This report gives the details about various methods of cleaning towards corrosion protection in aerospace industry.
12. 12. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 12 Chapter 2 Literature Survey
13. 13. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 13 2.1 CORROSION Corrosion is an irreversible interfacial reaction of a material (metal, ceramic and polymer) with its environment which results in its consumption or dissolution into the material of a component of the environment. Often, but not necessarily, corrosion results in effects detrimental to the usage of the material considered. Exclusively physical or mechanical processes such as melting and evaporation, abrasion or mechanical fracture are not included in the term corrosion. Corrosion is primarily associated with metallic materials but all material types are susceptible to degradation. Degradation of polymeric insulating coatings on wiring has been a concern in aging aircraft. Even ceramics can undergo degradation by selective dissolution. The fundamental cause or driving force for all corrosion is the lowering of a systems Gibbs energy. The production of almost all metals involves adding energy to the system. As a result of this uphill thermodynamic struggle, the metal has a strong driving force to return to its native, low energy oxide state. This return to the native oxide state is what we call corrosion and even though it is inevitable, substantial barriers (corrosion control methods) can be used to slow its progress toward the equilibrium state and it is this rate of the approach to equilibrium that is often of interest. This rate is controlled not only by the nature of the metal surface, but also by the nature of the environment as well as the evolution of both. Most corrosion processes involve at least two electrochemical reactions. A corroding surface can be thought of as a short-circuited battery; the dissolution reaction at the anode supplies electrons for the reduction reaction at the cathode. A short circuit is the electrical connection made by a conductor between the two physical sites, which are often separated by very small distances. Electrode potential difference between the reinforcing bars and electrolyte is the driving force for the charge transfer to occur. Their electrode potentials will change with the corrosion reaction rate until a stable or equilibrium state (Ecorr) is achieved. At this potential the anodic (ia) and cathodic (ic) current densities are opposite and equal and to the state (Icorr) achieved. It is graphical represented as a polarization curve (shown in fig 2.1). Deviation from the steady-state condition can be expressed by the electrode polarization potential, also known, as over-potential (a or c) where,
14. 14. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 14 a= E Ecorr c= Ecorr E Where, A=potential at anode C=potential at cathode Icorr =polarization at state achieved Ecorr=polarization at equilibrium state The study of corrosion processes involves the use of many of the same tools that are used by electrochemists studying batteries, fuel cells, and physical and analytical electrochemistry. The application of mixed potential theory to corrosion was originally presented by Wagner and Traud and discussed later in the Journal of the Electrochemical Society by Petrocelli. In 1957, Stern and Geary theoretically analyzed the shape of polarization curves providing the basis for the primary experimental technique (electrochemical polarization) used in electrochemical studies of corrosion. The formation of surface oxide films is critical in mitigating the rate of metal dissolution, so person study corrosion have much in common with those studying dielectrics for other purposes. It is these thin (< 10 nm) native oxide films that make the technological use of metallic materials possible by serving as barriers to dissolution. Traditionally, corrosion is classified into eight categories based on the morphology of the attack, as well as the type of environment to which the material is exposed. Uniform or general corrosion is the most prevalent type of corrosion but fortunately, it is predictable and can be controlled by various methods such as painting the surface or applying a layer of a sacrificial metal like zinc to steel. This sacrificial corrosion of the zinc surface layer to protect the underlying steel is actually a form of galvanic or bimetallic corrosion. In this case, like in a battery, we are using corrosion to our advantage. The surfaces of some metals (like aluminium, stainless steel, and titanium) are protected from uniform corrosion by an extremely thin oxide films that forms naturally. Many practical applications of materials depend on the presence of this
15. 15. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 15 protective oxide. We would not be able to use planes made from aluminium if it were not for this thin protective film. Unfortunately, this film can breakdown locally, resulting in forms of corrosion like pitting of aluminium plates, crevice corrosion of stainless steel fasteners, or stress corrosion cracking of pipes in nuclear reactors. In light of the thermodynamic basis for corrosion it is not surprising that costs associated with corrosion are high. Several studies over the past 30 years have shown that the annual direct cost of corrosion to an industrial economy is approximately 3.1% of the countrys Gross National Product (GNP). In the US, this amount rises to over $276 billion per year. From Fig. 1, the highest segments of the cost of corrosion are associated with utilities, transportation, and infrastructure. The Department of Defence alone has corrosion costs of $20 billion. Because of the significant economic, safety, and historical impact of corrosion on society and because corrosion of metals is an electrochemical process, it is also not surprising that the Corrosion Division is one of the oldest divisions within ECS and was established in 1942, but corrosion has been an important topic in the Society since 1903. Reviews of the early literature and history of the Division were prepared by Uhlig and Uhligs Corrosion Handbook is a good overall source of corrosion information for consultation purpose. 2.2 MECHANISM OF CORROSION Modern corrosion science was set off in the early twentieth century with the local cell model proposed by Evans and the corrosion potential model proved by Wagner and Traud. The two models have joined into the modern electrochemical theory of corrosion. They describe metallic corrosion as a coupled electrochemical reaction consisting of anodic metal oxidation and cathodic oxidant reduction. The electrochemical theory is applicable not only to wet corrosion of metals at normal temperature but also to dry oxidation of metals at high temperature. Metallic materials corrode in a variety of gaseous and aqueous environments. Here we restrict ourselves to the most common corrosion of metals in aqueous solution and in wet air in the atmosphere. In general, metallic corrosion produces in its initial stage soluble metal ions in water, and then, the metal ions develop into solid corrosion precipitates such as metal oxide and hydroxide .We will discuss the whole process of metallic corrosion from the basic electrochemical standpoint.
16. 16. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 16 2.2.1 ELECTROCHEMICAL ATTACK An electrochemical attack may be likened chemically to the electrolytic reaction that takes place in electroplating, anodizing, or in a dry cell battery. The reaction in this corrosive attack requires a medium, usually water, which is capable of conducting a tiny current of electricity. When a metal comes in contact with a corrosive agent and is also connected by a liquid or gaseous path through which electrons may flow, corrosion begins as the metal decays by oxidation. During the attack, the quantity of corrosive agent is reduced and in turn it completely reacts with the metal, becoming neutralized. Different areas of the same metal surface have varying levels of electrical potential and, if connected by a conductor, such as salt water, will set up a series of corrosion cells and corrosion will commence. All metals and alloys are electrically active and have a specific electrical potential in a given chemical environment. This potential is commonly referred to as the metals nobility. The less noble a metal is, the more easily it can be corroded. The metals chosen for use in aircraft structures are a studied compromise with strength, weight, corrosion resistance, workability, and cost Fig 2.1 Electrochemical attack
17. 17. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 17 balanced against the structures needs. The constituents in an alloy also have specific electrical potentials that are generally different from each other. Exposure of the alloy surface to a conductive, corrosive medium causes the more active metal to become anodic and the less active metal to become cathodic, thereby establishing conditions for corrosion. These are called local cells. The greater the difference in electrical potential between the two metals, the greater will be the severity of a corrosive attack, if the proper conditions are allowed to develop. The conditions for these corrosion reactions are the presence of a conductive fluid and metals having a difference in potential. If, by regular cleaning and surface refinishing, the medium is removed and the minute electrical circuit eliminated, corrosion cannot occur. This is the basis for effective corrosion control. The electrochemical attack is responsible for most forms of corrosion on aircraft structure and component parts.
18. 18. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 18 + Corroded End (anodic, or least noble) Magnesium Magnesium alloy Zinc Aluminium (1100) Cadmium Aluminium 2024-T4 Steel or Iron Cast Iron Chromium-Iron (active) Ni-Resist Cast Iron Type 304 Stainless steel (active) Type 316 Stainless steel (active) Lead-Tin solder Lead Tin Nickel (active) Inconel nickel-chromium alloy (active) Hastelloy Alloy C (active) Brass Copper Bronze Copper-nickel alloy Monel nickel-copper alloy Silver Solder Nickel (passive) Inconel nickel-chromium alloy (passive) Chromium-Iron (passive) Type 304 Stainless steel (passive) Type 316 Stainless steel (passive) Hastelloy Alloy C (passive) Silver Titanium Graphite Gold Platinum Protected End (cathodic, or most noble) Table 2.1 Galvanic series if metal
19. 19. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 19 2.2.2 BASIC PROCESSES The basic process of metallic corrosion in aqueous solution consists of the anodic dissolution of metals and the cathodic reduction of oxidants present in the solution: MM M2+ aq + 2e M anodic oxidation ------(2) Where M anodic oxidation. 2Oxaq + 2e M 2Red (e redox) aq cathodic oxidation -------(3) 2.3 CHEMISTRY OF CORROSION Common structural metals are obtained from their ores or naturally-occurring compounds by the expenditure of large amounts of energy. These metals can therefore be regarded as being in a meta-stable state and will tend to lose their energy by reverting to compounds more or less similar to their original states. Since most metallic compounds, and especially corrosion products, have little mechanical strength a severely corroded piece of metal is quite useless for its original purpose. Virtually all corrosion reactions are electrochemical in nature, at anodic sites on the surface the iron goes into solution as ferrous ions, this constituting the anodic reaction. As iron atoms undergo oxidation to ions they release electrons whose negative charge would quickly build up in the metal and prevent further anodic reaction, or corrosion. Thus this dissolution will only continue if the electrons released can pass to a site on the metal surface where a cathodic reaction is possible. At a cathodic site the electrons react with some reducible component of the electrolyte and are themselves removed from the metal. The rates of the anodic and cathodic reactions must be equivalent according to Faradays Laws, being determined by the total flow of electrons from anodes to cathodes which is called the corrosion current, Icorr. Since the corrosion current must also flow through the electrolyte by ionic conduction the conductivity of the electrolyte will influence the way in which corrosion cells operate. The corroding piece of metal is described as a
20. 20. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 20 mixed electrode since simultaneous anodic and cathodic reactions are proceeding on its surface. The mixed electrode is a complete electrochemical cell on one metal surface. Such electrochemical reactions are most common in acids and in the pH range 6.5 8.5 the most important reaction is oxygen reduction 2b. In this latter case corrosion is usually accompanied by the formation of solid corrosion debris from the reaction between the anodic and cathodic products. If solid corrosion products are produced directly on the surface as the first result of anodic oxidation these may provide a highly protective surface film which retards further corrosion, the surface is then said to be passive. 2.4 FACTORS THAT CONTROL THE CORROSION RATE Certain factors can tend to accelerate the action of a corrosion cell. These include: 1. Establishment of well-defined locations on the surface for the anodic and cathodic reactions. This concentrates the damage on small areas where it may have more serious effects, this being described as local cell action. Such effects can occur when metals of differing electrochemical properties are placed in contact, giving a galvanic couple. Galvanic effects may be predicted by means of a study of the Galvanic Series which is a list of metals and alloys placed in order of their potentials in the corrosive environment, such as sea water. Metals having a more positive (noble) potential will tend to extract electrons from a metal which is in a more negative (base) position in the series and hence accelerate its corrosion when in contact with it. The Galvanic Series should not be confused with the Electrochemical Series, which lists the potentials only of pure metals in equilibrium with standard solutions of their ions. Galvanic effects can occur on metallic surfaces which contain more than one phase, so that local cells are set up on the heterogeneous surface. Localised corrosion cells can also be set up on surfaces where the metal is in a varying condition of stress, where rust, dirt or crevices cause differential access of air, where temperature variations occur, or where fluid flow is not uniform. Stimulation of the anodic or cathodic reaction. Aggressive ions such as chloride tend to prevent the formation of protective oxide films on the metal surface and thus increase corrosion. Sodium chloride is encountered in marine conditions and is spread on roads in
21. 21. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 21 winter for de-icing. Quite small concentrations of sulphur dioxide released into the atmosphere by the combustion of fuels can dissolve in the invisibly thin surface film of moisture which is usually present on metallic surfaces when the relative humidity is over 60- 70%. The acidic electrolyte that is formed under these conditions seems to be capable of stimulating both the anodic and the cathodic reactions. In practical terms it is not usually possible to eliminate completely all corrosion damage to metals used for the construction of industrial plant. The rate at which attack is of prime importance is usually expressed in one of two ways: (1) Weight loss per unit area per unit time, usually mdd (milligrams per square decimetre per day) (2) A rate of penetration, i.e. the thickness of metal lost. If suitable water treatment with corrosion inhibitors is used a life of at least twenty years might be expected. This, of course, is ignoring the fact that at some time before the metal corrodes away the tubing may have thinned to a point where its required mechanical strength is not attained. When designing equipment for a certain service life engineers often add a corrosion allowance to the metal thickness, permitting a certain amount of thinning before serious weakening occurs. In a cooling water system the factors influencing the rate of attack are: a) The condition of the metal surface Corrosion debris and other deposits - corrosion under the deposits, with a possibility of pitting (severe attack in small spots) b) The nature of the environment pH - in the range of 4-10 corrosion rate is fairly independent of pH, but it increases rapidly when the pH falls below 4. Oxygen content - increase in oxygen concentration usually gives an increase in corrosion rate.
22. 22. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 22 Flow rate - increased water flow increased oxygen access to the surface and removes protective surface films, so usually increases corrosion, but can sometimes improve access for corrosion inhibiting reactants. Water type - very important, in general low corrosion rates are found with scale-forming (hard) waters. Aggressive ions which accelerate corrosion are Cl-, SO4 2- but quite complex interactions may occur between the various dissolved species in natural waters. 2.5 FORMS OF CORROSION There are many forms of corrosion. The form of corrosion depends on the metal involved, its size and shape, its specific function, atmospheric conditions, and the corrosion producing agents present. Those described in this section are the more common forms found on airframe structures. 2.5.1 Surface Corrosion Surface corrosion appears as a general roughening, etching, or pitting of the surface of a metal, frequently accompanied by a powdery deposit of corrosion products. Surface corrosion may be caused by either direct chemical or electrochemical attack. Sometimes corrosion will spread under the surface coating and cannot be recognized by either the roughening of the surface or the powdery deposit. Instead, closer inspection will reveal the paint or plating is lifted off the surface in small blisters which result from the pressure of the underlying accumulation of corrosion products. Filiform corrosion gives the appearance of a series of small worms under the paint surface. It is often seen on surfaces that have been improperly chemically treated prior to painting. 2.5.2 Dissimilar Metal Corrosion Extensive pitting damage may result from contact between dissimilar metal parts in the presence of a conductor. While surface corrosion may or may not be taking place, a galvanic action, not unlike electroplating, occurs at the points or areas of contact where the insulation between the surfaces has broken down or been omitted. This electrochemical attack can be
23. 23. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 23 very serious because in many instances the action is taking place out of sight, and the only way to detect it prior to structural failure is by disassembly and inspection. The contamination of a metals surface by mechanical means can also induce dissimilar metal corrosion. The improper use of steel cleaning products, such as steel wool or a steel wire brush on aluminium or magnesium, can force small pieces of steel into the metal being cleaned, which will then further corrode and ruin the adjoining surface. Carefully monitor the use of non-woven abrasive pads, so that pads used on one type of metal are not used again on a different metal surface . 2.5.3 Inter-granular Corrosion This type of corrosion is an attack along the grain boundaries of an alloy and commonly results from a lack of uniformity in the alloy structure. Aluminium alloys and some stainless steels are particularly susceptible to this form of electrochemical attack. The lack of uniformity is caused by changes that occur in the alloy during heating and cooling during the materials manufacturing process. Inter-granular corrosion may exist without visible surface evidence. Very severe inter-granular corrosion may sometimes cause the surface of a metal to exfoliate. This is a lifting or flaking of the metal at the surface due to delamination of the grain boundaries caused by the pressure of corrosion residual product build-up. This type of corrosion is difficult to detect in its initial stage. Extruded components such as spars can be subject to this type of corrosion. Ultrasonic and eddy current inspection methods are being used with a great deal of success. 2.5.4 Stress Corrosion Stress corrosion occurs as the result of the combined effect of sustained tensile stresses and a corrosive environment acting on the metal. Stress corrosion cracking is found in most metal systems; however, it is particularly characteristic of aluminium, copper, certain stainless steels, and high strength alloy steels (over 240,000 psi). It usually occurs along lines of cold working and may be trans-granular or inter-granular in nature.
24. 24. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 24 2.5.5 Fretting Corrosion Fretting corrosion is a particularly damaging form of corrosive attack that occurs when two mating surfaces, normally at rest with respect to one another, are subject to slight relative motion. It is characterized by pitting of the surfaces and the generation of considerable quantities of finely divided debris. Since the restricted movements of the two surfaces prevent the debris from escaping very easily, an extremely localized abrasion occurs. The presence of water vapour greatly increases this type of deterioration. If the contact areas are small and sharp, deep grooves may be worn in the rubbing surface. 2.6 CONSEQUENCES OF CORROSION The consequences of corrosion are many and varied and the effects of these on the safe, reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal. Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small. Some of the major harmful effects of corrosion can be summarised as follows: 1. Reduction of metal thickness leading to loss of mechanical strength and structural failure or breakdown. When the metal is lost in localised zones so as to give a crack like structure, very considerable weakening may result from quite a small amount of metal loss. 2. Hazards or injuries to people arising from structural failure or breakdown (e.g. bridges, cars, aircraft). 3. Loss of time in availability of profile-making industrial equipment. 4. Reduced value of goods due to deterioration of appearance. 5. Contamination of fluids in vessels and pipes (e.g. beer goes cloudy when small quantities of heavy metals are released by corrosion). 6. Perforation of vessels and pipes allowing escape of their contents and possible harm to the surroundings. For example a leaky domestic radiator can cause expensive damage to carpets
25. 25. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 25 and decorations, while corrosive sea water may enter the boilers of a power station if the condenser tubes perforate. 7. Loss of technically important surface properties of a metallic component. These could include frictional and bearing properties, ease of fluid flow over a pipe surface, electrical conductivity of contacts, surface reflectivity or heat transfer across a surface. 8. Mechanical damage to valves, pumps, etc, or blockage of pipes by solid corrosion products. 9. Added complexity and expense of equipment which needs to be designed to withstand a certain amount of corrosion, and to allow corroded components to be conveniently replaced. In light of the thermodynamic basis for corrosion it is not surprising that costs associated with corrosion are high. Several studies over the past 30 years have shown that the annual direct cost of corrosion to an industrial economy is approximately 3.1% of the countrys Gross National Product (GNP). In the United States, this amounts to over $276 billion per year. It is revealed that the highest segments of the cost of corrosion are associated with utilities, transportation, and infrastructure. The Department of Defence alone has corrosion costs of $20 billion [6] . Fig 2.2 Cost of corrosion in the US
26. 26. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 26 2.7 ANODIZATION Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts. The process is called "anodizing" because the part to be treated forms the anode electrode of an electrical circuit. Anodizing increases corrosion resistance and wear-resistance, and provides better adhesion for paint primers and glues. Anodic films can also be used for a number of cosmetic effects, either with thick porous coatings that can absorb dyes or with thin transparent coatings that add interference effects to reflected light. Anodizing is also used to prevent galling of threaded components and to make dielectric films for electrolytic capacitors. Anodic films are most commonly applied to protect aluminium alloys, although processes also exist for titanium, zinc, magnesium, niobium, zirconium, hafnium, and tantalum. Iron or carbon steel metal exfoliates when oxidized under neutral or alkaline micro-electrolytic conditions, the iron oxide (actually "ferric hydroxide" oxy hydrated iron oxide, also known as rust) forms minute anodic pits and large cathodic surface, these pits concentrate anions such as sulphate and chloride accelerating the underlying metal to corrode. Carbon flakes or nodules in iron or steel with high carbon content (high carbon steel, cast iron) may cause an electrolytic potential and interfere with coating or plating. Ferrous metals are thus commonly not subjected to anodization. Anodization changes the microscopic texture of the surface and changes the crystal structure of the metal near the surface. Thick coatings are normally porous, so a sealing process is often needed to achieve corrosion resistance. Anodized aluminium surfaces are harder than aluminium but have low to moderate wear resistance that can be improved with increasing thickness or by applying suitable sealing substances. Anodic films are not only much stronger and more adherent than most types of paint and metal plating, but also more brittle. This makes them less likely to crack and peel from aging and wear, but more susceptible to cracking from thermal stress.
27. 27. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 27 2.7.1 History Anodizing was first used on an industrial scale in 1923 to protect Duralumin seaplane parts from corrosion. This early chromic acid process was called the Bengough-Stuart process and was documented in British defence specification DEF STAN 03-24/3. It is still used today despite its legacy requirements for a complicated voltage cycle now known to be unnecessary. Variations of this process soon evolved, and the first sulphuric acid anodizing process was patented by Gower and O'Brien in 1927. Sulphuric acid soon became and remains the most common anodizing electrolyte. Oxalic acid anodizing was first patented in Japan in 1923 and later widely used in Germany, particularly for architectural applications. Anodized aluminium extrusion was a popular architectural material in the 1960s and 1970s, but has since been displaced by cheaper plastics and powder coating. The phosphoric acid processes are the most recent major development, so far only used as pre-treatments for adhesives or organic paints. A wide variety of proprietary and increasingly complex variations of all these anodizing processes continue to be developed by industry, so the growing trend in military and industrial standards is to classify by coating properties rather than by process chemistry. 1.7.2 Anodized Aluminium Aluminium and its alloys are anodized to increase corrosion resistance, to increase surface hardness, and to allow dyeing (colouring), improved lubrication, or improved adhesion. The anodic layer is non-conductive. When exposed to air at room temperature, or any other gas containing oxygen, pure aluminium is capable of self passivation by forming a surface layer of amorphous aluminium oxide 2 to 3 nm thick, which provides protection for some time but the thickness of this layer is usually not uniform. Aluminium parts are thus anodized to greatly increase the thickness of this layer for corrosion resistance. The corrosion resistance of aluminium and its alloys is significantly decreased by certain alloying elements or impurities: copper, iron, and silicon, so 2000, 4000, and 6000-series alloys tend to be most susceptible. Anodizing the parts not only enhances corrosion resistance but also their ability to retain dye which is not possible in case of untreated metal. Although anodizing only has moderate wear resistance, the deeper pores can better retain a lubricating film than a smooth surface would.
28. 28. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 28 Anodized coatings have a much lower thermal conductivity and coefficient of linear expansion than aluminium. As a result, the coating will crack from thermal stress if exposed to temperatures above 80 C. The coating can crack, but it will not peel. The melting point of aluminium oxide (2050 C) is much higher than pure aluminium (658 C). This at times makes welding more difficult. In typical commercial aluminium anodization processes, the aluminium oxide is grown down into the surface and out from the surface by equal amounts. So anodizing will increase the part dimensions on each surface by half of the oxide thickness. Anodized aluminium surfaces are harder than aluminium but have low to moderate wear resistance. Attempts are being made to overcome this problem of further improvement of thickness of anodized layer and sealing.
29. 29. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 29 2.7.3 Specifications STA NDA RD NOTES COMME NTS BS EN 2101:1991 Chromic acid anodising; Alloy category 1: Min film thickness 2.5 m Alloy category 2A: Min film thickness 1.5 m Alloy category 2B Min film thickness 1.0 m Sealing; Type A = Unsealed, Type B = Hot water sealed, Category 2 alloys shall preferable be dichromate sealed AEROSPACE SERIES BS EN 2284:1991 Sulphuric acid anodising; Class A Unsealed anodising Class B Sealed anodising Thickness class 1 12 to 25 m Thickness class 2 6 to 12 m Sealing as specified; Dyed aluminium hot water seal, Undyed aluminium hot water seal or dichromate seal. AEROSPACE SERIES BS EN 2536:1995 Hard anodising; Category 1 alloys < 1 % Cu : 30 m to 120 m film thickness Category 2 alloys 1% to 5% Cu: 30 m to 60 m film thickness. Note: Restrict thickness on splines & threads to 25 m Sealing is either hot water or dichromate seal AEROSPACE SERIES Table 2.2 Specifications of anodization in aerospace series
30. 30. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 30 2.7.4 MECHANISM OF ANODIZING PROCESS When a current is passed through an electrolyte in which an aluminium anode is employed, the negative charged anion migrates to the anode where it is discharged with loss of one or m o r e electrons. In aqueous solution, the anion consists parts of oxygen which chemically units with the aluminium and the result of the reaction depends on a number of factors, particularly nature of electrolyte , the consequent reaction product which are formed, and the operation conditions such as current potential, bath temperature, and time of treatment. In simple terms the following oxidation reactions at the anode can occur: 1. The anode reaction products may be soluble in the electrolyte. In this case metal is dissolved until the solution is saturated. This reaction takes place in some strong inorganic acids and bases. 2. The reaction product may be almost insoluble in the electrolyte and from a strongly adherent and practically non-conducting film on the anode. In this case film growth continues until 0 10 20 30 40 50 60 0 0.5 1 1.5 2 2.5 3 3.5 4 A' A B Fig 2.3 Change in thickness of the metal sheet A, anodized on both sides to the coating thickness indicated by the curve B, and A anodized on one side only, in sulphuric acid at 15 A/dm2 DC at 20C
31. 31. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 31 the resistance of the film prevents the current from reaching the anode. They can be formed in a number of electrolytes of which borate or tartarate solutions are the most common examples. Such films, formed at high voltage, find application in the production of the electrolytic condensers and for protection for very thin aluminium coating, example, those applied by vacuum deposition. 3. The reaction products may be sparingly soluble in the electrolyte and form a strongly adherent film that is non-conducting when dry, over the anode. In this case film growth takes place as above but is accompanied by dissolution of film at the surface. Pores are thus formed in the coatings that are wide enough to allow continuous access of the current to the metal. Film growth continues while the electrical resistance increases. When the rate of film growth has decreased until it is equal to the rate of dissolution of the film in the electrolyte, the film thickness remains constant. The maximum film thickness varies with the electrolyte and the operating conditions, especially the temperature which affects the dissolution velocity. The way in which the film thickness and thickness of the basic sheet vary with time is shown in the following figure. The curve A refers to the total increase in thickness of a sheet anodized on both sides to the coating thickness indicated by curve B, while the curve A refers to the dimensional change of a single surface. The coating reaches its maximum thickness in just almost 2 hours and it may be seen that upto this point, for every 3 microns of coating formed the metal surface retreats approximately to 2 microns and the exterior surface advances 1 micron. These are the conditions of industrial anodization process that are based chiefly on chromic, sulphuric or oxalic acid. 4. The reaction products may be moderately soluble. Under these conditions electro-polishing may be possible if a suitable electrolyte is used. Apart from the reactions considered there are a variety of less important possibilities, for example, where the reaction products may form loosely adherent, spongy or powdery deposits, as when the anodizing solutions become contaminated or when anodizing under special operations. A continuous adherent insoluble film, a few molecules thick, may render the metal passive [7] .
32. 32. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 32 2.7.5 Barrier Layer 2.7.5.1 Thickness It was shown as early as 1932 by Steoh and Miyata that the anodic oxide film consists of two layers, the porous thick outer layer growing on an inner layer which is thin, dense and dielectrically compact, and usually called the active layer, barrier layer or dielectric layer. This layer is very thin, i.e, usually between 0.1 and 2.0% of the total film, and its thickness depends on the composition of the electrolyte and the operating conditions. It has been established that the barrier layer formed in anodizing is of the nature of the natural oxide film formed in the atmosphere and that the barrier layer and porous films can also be distinguished coatings and on electro polished surfaces. In anodizing, the barrier layer is formed first and its thickness varies directly with the forming voltage. The barrier layer is non porous and conducts current only due to its thinness and faults in its skeleton. The outer layer, on the other hand, is micro porous and built upon a columnar structure. As long as no dissolution occurs in the electrolyte, the barrier layer is formed in a thickness of 14 A per volt. This is the theoretical maximum approached only in solutions in which little or no solvent occurs: thus, Holland and Sutherland obtained film thicknesses of 13 A per volt in 3% ammonium tartarate solution used in the protection of vacuum coated aluminium mirrors. Capacity measurements of barrier layers by Ginsberg and Kadan have given values of 14 A per volt for films formed in barrier layer electrolytes and 11.5 A per volt for barrier layers for porous anodic coatings [8] .
33. 33. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 33 The following table gives barrier layer and total thickness of oxide films produced by various treatments. Treatment Temperature (C) Barrier Layer Thickness (A) Total thickness ( m) Structure and composition of coating Dry air 20 10-20 0.001-0.002 Amorphous Al2O3 Dry air 500 20-40 0.04-0.06 Amorphous Al2O3 + - Al2O3 Dry oxygen 20 10-20 0.001-0.002 Amorphous Al2O3 Dry oxygen 500 100-160 0.03-0.05 Amorphous Al2O3 + - Al2O3 Humid air 20 4-10 0.05-0.1 Boehmite + Hydragillite Humid air 300 8-10 0.1-0.2 Undetermined Boiling in water 100 2-15 0.5-2.0 Boehmite Autoclavi ng in water 150 About 10 1.0-5.0 Boehmite Chemical oxidation 7-100 2-8 1.0-5.0 Boehmite + Solution anion (e.g.CrO4 ,PO4) Normal anodizing 18-25 100-150 5-30 Amorphous Al2O3 + solution anion Hard anodizing +6- -3 150-200 150-200 Amorphous Al2O3 + solution anion Barrier film anodizing 50-100 300-400 1.0-3.0 Crystalline Al2O3 + Amorphous Al2O3 + solution anion Chemical polishing 50-100 About 5 0.01-0.1 Boehmite +Solution anion Electro polishing in H3PO4 butyl alcohol 50-60 50-100 0.1-0.2 Al2O3 (structure not determined) + solution anion Table 2.3
34. 34. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 34 2.7.5.2 Effect of Operation Conditions On the Barrier Layer The way in which anodizing time affects the thickness of the barrier layer has already been discussed. The way in which other variables affect the unit barrier thickness, i.e, the thickness per volt of applied potential is shown below. Electrolyte Type The unit barrier thickness as shown in the following table which also gives other dimensions of these coatings, referred to in greater detail below. Electrolyte Conc . Temperature (C) Unit barrier Thickness (A/volt) Pore thickness (A/volt) Wall diameter (A) Phosphoric acid Oxalic acid Chromic acid Sulphuric acid 4 2 3 15 25 25 40 10 11.9 11.8 12.5 10.0 11.0 9.7 10.9 8.0 330 170 240 120 Table 2.4 Temperature of Electrolyte The effect of temperature on the unit barrier thickness at different voltage is shown in the following graph. It is seen that the effect of voltage is negligible. Increasing temperature may decrease the unit barrier thickness slightly due to increased rate of dissolution of the oxide, but under, some conditions the reverse has been observed.
35. 35. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 35 Fig 2.4 Effect of voltage and temperature on unit barrier thickness for coatings on 99.99% aluminium formed in 15% sulphuric acid Current Density The following graph shows the relation between voltage, current density and temperature during sulphuric acid anodizing. Increase in temperature decreases the minimum voltage at which the current density rises steeply with the forming voltage. However, the current density has little effect on unit barrier thickness 8.8 9 9.2 9.4 9.6 9.8 10 10.2 0 10 20 30 40 50 60 70 80 unitbarrierthickness (angstorm/volt) Bath Temperature (C) 0C20C40C60C 70C 0 20 40 60 80 100 120 0 5 10 15 20 25 300 deg
36. 36. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 36 centration of Electrolyte The effect of electrolyte concentration is seen in the following graph. At constant voltage and temperature and with the use of very low concentrations the unit barrier thickness approaches a maximum of 14A/volt, as at this concentration the solvent action is low. Increasing the concentration causes a drop in the unit barrier thickness that reaches a minimum between 35 and 65% (weight %) sulphuric acid. This is followed by a marked increase upto 90% where it changes sharply to an almost negligible value. The decrease in unit barrier thickness at higher concentration is by no means related to the rate of dissolution, nor is it directly related to the degree of dissolution of sulphuric acid as related to the electrical conductivity, suggesting that some other influences barrier thickness at high acid concentrations. In other electrolytes, such as chromic, oxalic or phosphoric acid, the barrier thickness is influenced by the same factors to a very similar extent. 0 2 4 6 8 10 12 14 16 0 20 40 60 80 100 120 Y-Values Fig 2.6 Relationship between concentration of sulphuric acid and unit barrier thickness of coating formed on 99.99% aluminium at 20C and 15V Fig 2.5 Voltage, current density and temperature relationship during coating of 99.99% aluminium in sulphuric acid
37. 37. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 37 2.7.6 Porous Layer 2.7.6.1 Porosity As well as the thickness, the porosity of the coating varies with the dissolution velocity and the conditions and rate of film growth, and these depend on the operating conditions and the type of electrolyte. As far as the last is concerned, it is probable the pH of the solution is the most important. An example of this is seen most strikingly when aluminium is anodized in phosphate solutions. While a phosphoric acid electrolyte gives a thick and extremely porous anodic oxide coating, a buffered phosphate solution on the other hand gives a non-porous barrier film the phosphate content of which is an integral part of the film and is proportional to its thickness. Due to the effect of dissolution, the outer layer of the coating have the greatest porosity. Examination by electron microscope shows the presence of pores in the striated structure. Edwards and Keller also found vertical lines 6 x 10-9 inches apart near the metal interface, which they believe locate the pore centres from which the coating grows. The pores are very absorptive and it was determined that when a film, formed in sulphuric acid with a volume of 15 ml/sq m was boiled in a 1% potassium dichromate solution for one hour, the coating took up 0.48 gm of chromium per sq m, in other words the dichromate content of the 140 ml of solution or 10 times the volume of the coating, was absorbed and concentrated in the pore surface as fresh solution continued to diffuse in to the pores. The effect of anodizing temperature on the absorption capacity of the film, i.e, on its porosity, is shown in following graph.
38. 38. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 38 The largest pore diameter of coating produced industrially is found in the phosphoric acid coating that is used as a base for electro deposition. Next come the oxalic acid films particularly films produced by the AC process. Thus it was found by direct electron microscope examination that thin DC sulphuric acid films give approximately 800 pores/nm2 (pore diameter 0.015 nm; porosity 13.4%). While DC oxalic acid coating gives 60 pores/ im2 (pore diameter 0.075 nm; porosity 8%, amended to 12% to allow for pore sections not appearing on the surface due to their direction). No pores have been found on the barrier layer, and on examining films formed in ammonium borate and disodium phosphate, found on determinable structure under the electron microscope. Chromic acid coatings, due to their relatively low solubility are more closely allied to barrier films and have a smaller pore diameter. The total porosity of coatings formed in chromic, sulphuric and oxalic acid coatings has been variously estimated from 12-30 %. More detailed investigations on the mechanism of anodic oxidation show that the number of pores and their volume are largely dependent on the forming voltages. 0 10 20 30 40 50 60 70 0 5 10 15 20 25 30 35 40 bathvoltage(volts) temperature (C) Fig 2.7 Effect of bath temperature on water absorption of coating, i.e., on porosity. Film 10 microns thick produced in 20% sulphuric acid at 20 A/sq ft subsequently immersed in water for 30 mins
39. 39. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 39 While the ratio of effective metal surface apparent surface is extremely low. i.e., the effective current density is much larger than the calculated values. The pore area is extremely large. True values for the current density of porous film formation cannot easily be determined. Values based on the average pore diameters do not take into account the considerable decrease pore diameter at the lower layers or the electrical relationship through the dielectric layer or the relationship between current density and voltage during film formation at constant current density is shown in the following graph. 2.7.6.2 Mechanism of porous film growth When aluminium is made anodic in the anodizing electrolyte, it will depend on the operating conditions, i.e., voltage, solubility of the reaction products, concentration, temperature, etc. There are two possibilities for a reaction in a sulphuric acid electrolyte, in one of which the O2- ion and water react directly with the aluminium, while in the other the aluminium 0.1 A/sq dm 0.5 A/sq dm 1 A/ sq dm 2 A/sq dm 5 A/sq dm 10 A/sq dm 0 20 40 60 80 100 120 0 20 40 60 80 100 120 140 bathvoltage(volts) Time (seconds) Fig 2.8 Bath voltages and constants current densities in relation to the treatment time in 2 % oxalic acid, DC at 17-18C
40. 40. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 40 sulphate first formed is hydrolysed to the hydrate. Once oxide has been formed on the metal surface, the anion can no longer make contact with the aluminium. The oxidation reaction is given empirical by the simple equation:- 2 Al + 3 O Al2O3 + energy The above reaction takes place at the metal-oxide interface. 2.7.7 Coating ratio A useful concept in determining the course of anodic oxidation is the coating ratio. This term represents the weight of coating divided by the weight of aluminium reacting (the last being the combined weight of metal converted into oxide plus that going into the solution) 200 interuppted dye adsorbed normal 0 50 100 150 200 250 0 20 40 60 80 100 120 140 thickness(mil) 1mil=25microns time (minutes) Fig 2.9 Effect of anodizing time on film growth and on the dimensions of the part being anodized
41. 41. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 41 Assuming that the coating is composed of Al2O3 the coating ratio has a theoretical ratio of 1.89. In practice the anodic coatings nearly always contain some of the solution anion. In the case of sulphuric acid anodizing on anodizing on 99.95% aluminium, approximately 12- 14% SO3 is found in the coating which gives a maximum coating ratio of 2.2. From the foregoing discussion on the effect of operating variables, it follows that, other condition being constant, the coating ratio will decrease with time and increased by reducing the bath temperature and acid concentration or by increasing the current density and voltage. It can also be increased by addition of a certain amount of oxalic acid. Decrease of coating ratio is approximately linear with time at constant current density. The voltage rises as the coating thickness increases and this reflects increasing dissolution of the coating during its growth associated both with a larger active surface area with progressive dissolution in the pores and increase in the local temperature due to the higher voltage required as the coating grows in thickness. The effect of increasing the current density is to speed up the rate of growth. The effect of current density will last the whole course of normal anodizing and becomes even more pronounced in time if the bath temperature has a very pronounced effect on increasing the 2S M18 245 T3 755 T6 99.95 Al 615 T6 1.54 1.56 1.58 1.6 1.62 1.64 1.66 1.68 1.7 0 50 100 150 200 coatingratio metal removed (milligram) Fig 2.10 Coating ratios for various alloys treated as anodes in 15% H2SO4 at 1.1C with a current density of 2.5A/dm2
42. 42. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 42 coating ratio. Thus at 10C, which is a temperature commonly used in hard anodizing, the effect of anodizing in reducing the coating ratio is negligible at current densities as low as 24 amp/ sq. ft, while at current densities above 48 amps/ sq. ft prolonging the treatment causes a progressively steep rise in the coating ratio. This rise can be explained as being due to a decrease in solution rate within the pore channel due to build-up of the solution product. The excess of dissolution products may be pictured as due to the rapid dissolution at the high temperatures obtaining at the pore base (estimated at 125 C), which cannot be dealt with by the diffusion rate further up the pore in cooler solution. For the same reason, it is noteworthy that increase in the coating ratio with time at low temperatures and high current densities is inevitably associated with a steeply rising voltage, and in practice, these conditions therefore present serious disadvantages. Different alloys behave rather differently in cold sulphuric acid electrolytes. The coating ratio for commercial aluminium tends to rise with time at relatively low temperatures, due possibly to stronger initial dissolution. Anodic oxide films that contain heavy metals dissolve rapidly from the surface and it is difficult to obtain uniform coatings of any thickness. Sometimes the coating ratio is used as a control method for evaluating the efficiency of the anodizing process and production work.
43. 43. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 43 EFFECT OF OPERATING CONDITIONS ON PROPERTIES OF COATING BATH Change in operating conditions Limitng film thickness Hardness Corrosion resistance porosity voltage Temperature increase Current density increase Reduction in time -- Decrease in acid concentration Use of AC Increase in homogeneity of alloy structure Use of less aggressive electrolyte = increase = passes through a maximum = decreases Table 2.5
44. 44. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 44 2.7.8 ANODIC OXIDE COATING COMPOSITION The mechanism of anodic oxidation is very complex and is still largely controversial. The theories developed are numerous and complicated and hence will be dealt with in a very brief manner. As from the discussion of aluminium oxide films in general, the compositions of coatings depend largely on the electrolysis employed and differing and different workers have reached sometimes, contradictory conclusions. Thus, Bengough and Sutton found 0.4-0.7 chromic acid in the coatings produced by chromic acid while Pullen and Scot, working with coatings sealed in water found them to consist of almost entirely anhydrous aluminium oxide Al2O3 with less than 0.1% of chromium. At the same time the latter authors found the sealed sulphuric acid coatings to have 13% SO3 and the oxalic acid coatings to contain about 3% of (COOH) 2. The composition coated by Scott for the sealed sulphuric acid coating is: Aluminium oxide - 72% Water - 15% Sulphur trioxide - 13% These measurements were made with thick porous coatings detached from the base metal, by Scott obtained almost identical results some 25 years later using thin coatings and a different analytical technique. According to Mason the sulphate content of the normal sulphuric coating is between 13% - 17% is higher at lower temperatures of operation and increases with current density. Spooner has given the following composition: Compound Unsealed coating Water sealed composition Al2O3 78.9% 61.7% Al2O3.H2O 0.5% 17.6% Al2(SO4)3 20.2% 17.9% H2O 0.4% 2.8% Table 2.6
45. 45. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 45 These figures are equivalent to SO3 contents of 14.2 and 12.6% respectively but the H2O contents are 0.5 and 5.4%. Edwards and Keller have reported water contents of 1.0% and 6.0% in sulphuric acid and anodized coatings. These figures suggest a possibility of a partial hydration of the coating. Phillips reports water content in coatings produced in oxalic acid equivalent to the formula 2 Al2O3.H2O. The use of radioactive tracer methods for estimating sulphate by using sulphuric acid incorporating some of the isotopes of S35 has enabled the role of sulphate to be examined with much more precision. Thus Brace and Baker have detected sulphate during the first few seconds of anodizing at about 15% slowly falling as anodizing proceeds, and it even reaches 1.5% during chromic anodizing. E.Raub and his co-workers have more recently applied the same techniques to quiet a detailed examination of sulphate incorporation from coatings prepared in electrolytes of very low sulphate content based on sulphur sulphosalicyclic acid or maleic acid. They have demonstrated that although the electrolytes contained only 0.35% and 0.5% of sulphuric acid, this was essential for the proper operation of the process and the coatings was found to contain from 5-10% of the sulphate ion. Anhydrous aluminium oxide is very hygroscopic even when heated to red heat, and the electrolytically produced coating is like a gel in equilibrium with the vapour pressure. Change in weight of the anodized aluminium rises steeply with humidity above about 70% relative humidity and on a 20 micron coating the weight may rise to as much as 600mg/sq cm. As might be expected, water uptake by sealed coatings is markedly lower at the higher humidities, although at lower humidity there is often little difference. On heating, anodic oxide films lose ionic conductance, due probably to a decrease in aluminium ion, until the film is undistinguishable from an amorphous film formed in dry oxygen. As electric conductance of the film is proportional to its moisture content, this property has been utilized has been utilized in an instrument designed to determine humidity. Most investigators agree that the coating consists mainly of anhydrous aluminium oxide which is either amorphous or in the - Al2O3 or 1 - Al2O3 state, though there is disagreement concerning these forms. - Al2O3 is intermediate in formation temperature
46. 46. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 46 between the two - Al2O3 stages while 1 - Al2O3 is the form produced on heating the monohydrate - Al2O3.H2O to 650C. Ruziewiez observed changes in the photoluminescence of anodic films when heated at 450C and 680C. Franklin found at least 3 types of oxide in films formed in boric acid-borax solution: 1. Anhydrate layer at the outside of the film 2. Irregular patches of crystalline 1 - Al2O3 within the coating. 3. Amorphous oxide as bulk of the film On the other hand there is now a great deal of support, example for the view that the proportion of crystalline oxide in the film increases with the film thickness while the effect of pre-treatment on the structure of anodic oxide coatings on aluminium cannot be ruled out. Thus there is some evidence that absorbed chloride, present example, on the aluminium foil that has to be etched before use in capacitators increases the amount of boehmite present after anodizing in barrier film electrolytes compared with foil that is etched in hydrofluoric acid. In practice, this effect is removed by rinsing in hot water containing silicon after hydrochloric acid etching. 2.7.9 Comparison of AC and DC anodization In order to overcome the problem of liberated oxygen forming a passive layer on the surface on which anodization is to be done, it is desired to use AC instead of DC. The use of AC is done not with the change of polarities but in a pulsated mode DC so that the passivation of the surface is reduced. In industry AC has also been used successively but anodized surfaces with AC have certain inherent deficiencies. The film produced by AC is more transparent. The advantage of AC is due to the lower of film in comparison with DC films obtained at lower current densities. AC films can be dyed more deeply and more uniformly than DC films. DC anodizing at low voltages could also obtain most of the advantages of AC processes. But in some cast materials, more uniform dyeing can be obtained by using AC process due to the very efficient degreasing action of AC.
47. 47. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 47 No sound film, thicker than about 12 microns, could be obtained with AC and therefore this process is not suitable when a high degree of resistance to pitting corrosion is required. The pore numbers of Ac films are slightly higher than for equivalent DC films, butt AC films have a tendency to hydrate in sealing to a much greater extent. The natural colours of AC films are yellowish. The depth of the colour is a function of the film thickness only and not of other anodizing conditions. The colour is greatly diminished by boiling water sealing and is greatly intensified by copper or ferrous ions in anodizing bath. Abrasion and corrosion resistances of AC films are much lower than those of equivalent DC films. When the comparison is made is made between films thicker than 6 mm at low temperatures and high current densities, the comparison becomes more favourable to AC films. 1.8 PROPERTIES AND TESTS OF ANODIC OXIDE COATINGS As discussed previously, the properties of the coatings obtained by the various anodizing processes may vary considerably, and depending on the specific application for which the work is to be treated, it is often possible by varying the solution, the operating conditions, the after-treatment, or even the composition of the basic metal or alloy, to obtain improvement in the properties aimed at. This section comprises of discussing the physical and chemical properties of anodic coatings along with the testing methods that have been employed, certain of which might with advantage be incorporated in routine control and inspection in aerospace practices, where being specific is particularly important. 2.8.1 Apparent density The apparent density (specific gravity) of anodic oxide coatings may vary within quiet appreciable limits depending upon the operating conditions and the basic metal. The
48. 48. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 48 variations are due to differences in porosity and in foreign inclusions in the film, hence the term apparent density is more appropriate and now generally preferred. The following table shows apparent densities of sulphuric acid anodic oxide coatings with anodizing time and basic metal composition. Anodizing Time (Mins) Apparent Density (gm/ m3 ) 99.99% Al Al -3% Mg Al-Mg-Si Al-Cu-Mg 5 3.3 3.4 3.5 2.9 10 3.2 3.3 3.1 2.4 20 3.0 2.6 2.8 1.9 30 3.0 2.5 2.4 1.8 40 2.6 2.5 2.3 1.6 50 2.4 2.3 2.4 - 60 2.2 2.3 2.3 - Table 2.7 2.8.2 Coating thickness The thickness of coatings normally produced by the different anodizing processes has been described earlier. As has been seen, the increase in film thickness is not linear with the treatment time, but a maximum, or limiting, film thickness may often be reached when equilibrium is established between the rate of film growth and the rate of dissolution of the film in the electrolyte. The operating time is often critical. However, in that the metal will continue to decrease in thickness after the maximum film thickness has been obtained, while in case of some alloys, example, certain Al-Mg and Al-Mg-Zn alloys, there may even be an actual decrease in film thickness after the maximum is reached.
49. 49. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 49 In general, the limiting thickness and the rate of film growth increase with rise in current density and pH of the solution, with vigorous agitation and with greater homogeneity of the alloy, and decrease with rise in temperature and the presence of heterogenous phases of alloying constituents that accelerate the dissolution of the film in the electrolyte. The most effective method of producing thicker coatings is to use low temperatures. In the case of dielectric non-porous films, which are practically insoluble in solution, such as films produced in the boric acid electrolyte, the film growth of the barrier type film ceases when the breakdown voltage is equal to the voltage applied. In most cases anodizing first increases the dimensions of the work and then reduces them again after reaching a maximum. In general, the sulphuric acid film itself has a volume approximately 1.5 times that of the metal from which it is formed but this ratio is slightly higher in oxalic acid anodizing and hard anodic films may be 2.0 times the volume of the metal removed during their formation. In practice, it is often desirable to measure the film thickness periodically, both as a check on the solution and on the quality of the work, as the thickness of the coating influences its resistance to corrosion and wear. This is of particular importance where the work is to be dyed or where close dimensional tolerances have to be obtained together with adequate protection. It is of course, essential where specification is to be maintained. DETERMINATION OF COATING THICKNESS Numerous methods have been suggested, but no technique that is both simple and accurate has been evolved suitable for all types of techniques that is both simple and accurate has been evolved suitable for all types of techniques. Methods for determining the thickness of anodic coatings in routine inspection should preferably not destroy the film and at least should not affect the basic metal. Some film thickness meters are now accurate and dependable but the referee methods in cases of dispute inevitable involve destruction of the coating. a) Direct Microscopic Measurement This is an adaptation of the method for determination of thickness of mirrors and can be only used for transparent films. It is a non-destructive method for measuring coating
50. 50. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 50 thickness. A microscope is employed whose adjustment includes a micrometer device. The microscope is first focussed on the surface of the coating, then on the coating-metal interface. The difference measured on the micrometer device gives the optical film thickness that must then be multiplied by the refractive index, which is 1.59 for unsealed and 1.62 for hot-water sealed films. The accuracy of this method increases with the thickness of the coating and the magnification of the microscope (should be upto 1000 dia). Modification of this technique is to immerse the article in oil which causes a reduction in the effective refractive index. The air-film interface may be focussed with greater accuracy by rubbing the surface lightly with pencil. If the surface is highly reflective, the shadow of the pencil mark is taken as the metal-coating interface, or the distance between the pencil marking and its mirror image is determined, being equivalent to twice the optical thickness. b) Eddy current measurement The most reliable non-destructive methods for measuring coating thickness are those carried out with meters based on the eddy current principle which are designed so that the strength of a high frequency current flowing through the search coil is dependent on the distance of the coil from a conducting surface. Out of the earliest commercial instrument was the Isometer, in which the test head is a small coil energized by a high-frequency oscillator. The associated magnetic field induces eddy currents in the basic metal. The depth of penetration of the current is inversely proportional to the square root of the frequency and directly proportional to the conductivity of the basic metal. Thus, when the conductivity of the coating differs from that of the basic metal the coating thickness (i.e., the distance between the probe and the basic metal) is linear to the output of the amplifier, which is measured on a dial. In order to prepare calibration curves for the specified aluminium alloy, non-metallic foil of known thickness is placed on the uncoated metal. Eddy Current instruments have been used for several years, principally as flow detectors, as alloy sorters or resistivity-measuring devices. The majority of instruments require a zero- setting on uncoated metal of the same composition or alloy type as the anodized metal to be
51. 51. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 51 measured, and standardizing on coatings of accurately known thickness, which are sometimes supplied as plastic films. The commercial instruments vary in accuracy, stability and dependability with the course of time. Defects of the earlier models were associated with zero drift requiring very frequent rechecking, a high sensitivity to temperature changes, a response that was not always linear and probes whose characteristics are not always stable. c) Micro-section The traditional method for determining the anodic coating thickness without equivocation is the preparation of a standard metallurgical micro-section that can be viewed with a high- power microscope fitted with a calibrated micrometer eye-piece, or which is equipped with a projection screen on which direct measurements at a known magnification can be made. It is destructive, very time consuming, allows only a small portion of the surface to the surface to be examined and requires some skill and experience because of the tendency for the edge of the coating to bevel or chip. In fact, in inexperienced hands it can be less certain than the eddy current method. The British Non-Ferrous Metals Research Association has given the following guidance: Sections shall be cut using a fine jewellers saw to avoid deformation and blurring of cut edges. The cut edges of anodic coatings require support to retain a true profile during polishing and for ease of differentiation between coating and mounting medium. This may be achieved in a variety of ways, the following method being recommended. The anodized surface of the specimen should be tightly wrapped with a single layer of smooth aluminium foil and folded at one end to retain in place. The wrapped specimen is mounted using a suitable thermosetting resin. Fine particles of resin powder should be first packed around the specimen to ensure the complete filling of voids and the pores finely filled using the coarser resin particles. The anodized specimen should be perpendicular to the face of the completed mount, a deviation of 100 introducing, however, only an error of 2% in the thickness. After mounting, the specimen must be free from voids between the section and the mounting medium.
52. 52. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 52 Mounted specimens are ground on emery paper using water or white spirit lubrication and the minimum pressure applied to avoid bevelling of the surface. Initial grinding should employ 100 or 180 grade emery to reveal the true specimen profile and remove any deformed areas. A final polish for 2-3 minutes on a rotating wheel charged with 4-8 micrometer diamond paste particles and white spirit lubrication should suffice to remove emery scratched for final examinations. Where very soft aluminium substrates are being prepared emery particles may become embedded during grinding. This may be minimised by totally immersing emery papers in lubricant during grinding or by using a copious flow of lubricant. If emery particles do become embedded they may be removed by applying a short, light hand polish with metal polish after grinding and before diamond finishing. The most convenient microscope magnification for viewing the section is 1000 because 1mm on the screen is equal to 1 micron film thickness. The accuracy with which the coating can be measured is generally about + 0.5m, and the average of several determinations is taken. It should be noted in passing that most other method of film thickness determination are calibrated by means of standards that have, or should have been measured by micro-section. They cannot therefore be any better in absolute accuracy than the micro-section method, and determinations of density are subject to proportionate errors that obviously become greater when applied to thinner films. The most accurate calibrations require the techniques of interferometry and expensive equipment. 2.8.3 Porosity There are semantic problems associated with applying the concept of porosity to anodic oxide coatings because here we are dealing with materials whose nature is inherently porous, while at the same time superimposing on this secondary concept borrowed from the terminologies of electrodeposits which is related more to discontinuities in an essential homogenous medium. These two properties are sometimes distinguished by referring to them as micro and macro porosity, but this is not altogether ambiguous because the micro
53. 53. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 53 porosity is on a scale far below the reach of any microscope and can only be imperfectly seen at magnifications approaching 100,000 with an electron microscope, while macro porosity can be associated with a number of factors, including inter-metallic constituents, which are of truly microscopic dimensions. The porosity, which is part and parcel of anodic oxide coatings, formed in acid electrolyte arises from dissolution of the coating in the electrolyte so that continuous film formation becomes possible, and the dimensions of these pores are a function of the electrolyte, the operating conditions and the thickness of the coating. The nature and magnitude of this porosity is important because it affects the resistance to abrasion, to corrosion, the case with which a coating may be dyed or otherwise impregnated, and the efficiency with which it can be sealed. In very thin anodic coatings the pores can be produced to an extremely uniform size, example, to within + 10%, and much of our knowledge of pore structure has come from observation of such films, while in practice they have been used for the filtration of gaseous colloids or colloidal suspensions. It has also been shown that micro porosity can also be influenced by the texture of the metal surface, decreasing with the smoothness of the surface, and being less in electro-polished surfaces than in mechanically polished surfaces after anodizing. While micro porosity is closely linked with the physical properties of the coating as noted above, the continuity or macro-scale features are related to corresponding features or faults in the basic metal, or to extreme operating conditions, and these may be detrimental to appearance or corrosion resistance. The quantitative estimation of porosity will vary to an extent with the method of definition or test because the dimensions involved are such that they will admit some molecules and not others and so that gaseous absorption or theoretical calculation of the void space may not correspond with what can be absorbed in the nature of a solid pigment. Lead Acetate Absorption A number of methods have been devised which are based on impregnation of the anodic coating. In one such test, the specimen is anodized, dipped for 10 minutes in distilled water
54. 54. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 54 in order to remove traces of electrolyte, dried at 110C for 30 minutes, and weighed. Subsequently the specimen is immersed in lead acetate solution for 2 hours, again washed in distilled water, dried and re-weighed. The gain in weight caused by impregnation with lead acetate is determined and porosity is calculated. Toluene Absorption (real density) Immersing coatings in toluene and using Archimedes principle, to measure the density as opposed to the apparent density, pore volume can be calculated. With unsealed coatings formed on 99% Al in sulphuric acid, the density was found to be 2.96 which corresponded to a pore volume of 15.8% while after sealing the real density became 2.65. Using an alloy containing 4.5% Cu, 1.5% Mg, a pore volume of 47% was found. In an oxalic acid electrolyte there was very little change in real density with rise in anodizing temperature and only a slight increase in porosity, which is different from sulphuric acid where the role of temperature is very important. Dielectric constant Making measurements of the apparent density and the apparent dielectric constant and then assuming values for the real density and the real dielectric constant, which is assumed to be 2.95 and 8.70 respectively, the porosity of coatings formed in sulphuric acid can be estimated. The porosity of oxide formed is 15% in sulphuric acid at 4 A/sq dm, 30C for 30 minutes is 24-26% while the porosity of film formed at 1C is 10-14%. When aluminium is anodized at more than 20C a rapid increase in porosity is observed. The results can be confirmed by measuring the amount of transformer oil that can be absorbed by unsealed coating. 2.8.4 Adhesion The adhesion of the oxide coating is normally much better than that of the electrodeposits but the film tends to be weak vertically to the surface, i.e. , the film is apt to crack transversely to the direction of rolling. When bent, the coating cracks in parallel lines but will not strip off as electrodeposits do. Care must be taken, however not to leave anodized
55. 55. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 55 work in the electrolyte when the current is switched off as this tends to loosen the film and to decrease adhesion. In general, the adhesion of the film increases with increasing temperature, acidity and the use of DC, as well as low current densities and longer treatment times. As the coating is an integral part of the surface, no adhesion test is used for anodized aluminium in normal circumstances. 2.8.5 Sealing Efficiency In spite of the profound effects of sealing on properties and performance of anodic oxide coatings, there has been a need for a good infallible test since many years to indicate how efficiently sealing has been performed. The most frequently employed property has been the increased resistance to chemical attack and this can be judged visibly by the appearance of the coating, or by the extent to which it absorbs a dye-stuff. After attack, it can also be assessed quantitatively by a photometric measurement of the depth of this dyeing, or finally with more certainity by measuring the loss of weight. Recently impedance measurements have become popular. The various methods that have been employed as discussed below. Certain tests have been used in production control to assess the efficiency of sealing. It is important to note that sealing tests (as well as those which measure electrical breakdown of the sealed anodic oxide coating) may give misleading results on coatings that have been stored for some time before testing. The changes that take place in the film are so profound that after a period of 7 to 8 weeks of storage, it may be impossible to distinguish between well and badly sealed films by some tests. Dye Stain Test These are commonly employed when one needs to know whether sealing has been performed or omitted. At one time this was all that one needed to know and they were the sole tests of sealing, but it is now known that failure to absorb dye represents only the initial stages of the sealing operations and they are therefore regarded as resistance-to-marking tests that may be specified where a surface is intended for mild indoor service.
56. 56. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 56 The acid violet test is a commonly used test. Two separate drops of a dye solution is made by dissolving 1gm C.I Acid Violet No. 34 or a corresponding dyestuff in 50ml distilled or deionised water , are applied at room temperature to the anodized surface and allowed to stand for 5 minutes. The test piece is then rinsed in running water and the test area swabbed for 15 seconds with cotton wool in a detergent solution (1gm sodium dioctylsulphosuccinate in distilled or deionised water, allowed to stand for 12 hours and made upto 100ml). The test piece is rinsed and dried with filter paper without rubbing. The sample passes the test if no mark remains. Acidified Sulphite Test In the sodium sulphite sealing test the specimen is immersed in a solution containing 10gm/lt anhydrous sodium sulphite adjusted to pH 3.75 with glacial acetic acid and then to pH 2.5 with 5N sulphuric acid. The solution is kept at 90-98C and the specimens are immersed for 30 minutes. A numerical rating system is based on visual standards and ranges from 5 for a perfect specimen with little or no change in appearance and no bloom to 0 for removal of the coatings. In practice, a rating of 3, which is accorded to a surface with a light bluish tinge and light bloom, is usually considered acceptable, while a rating of 2, corresponding to a blue-grey surface with moderate bloom is normally considered to be insufficient for acceptance. On bright-anodized materials, there is little visual difference between ratings 5 and 0 (coating removed) and a simple test with a flashlight and battery must be performed to check whether any coating is left. 2.8.6 Corrosion CASS Test The letters CASS stands for copper-accelerated acetic acid-salt spray test which is operated at a higher temperature than the acetic acid-salt spray test and includes a proportion
57. 57. SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014 DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 57 of cupric chloride which is perhaps the best additive for promoting rapid attack of exposed aluminium. The solution used for spraying is made by dissolving 50 + 0.02gm of cupric chloride in water containing less than that 100ppm of total solids or having a conductivity of less than 0.002 s/m, and diluting to 1 litre. Glacial acetic acid is added to adjust the pH to 3.2 + 0.1. The cabinet in which the test is carried out in frequently made from Perspex but may be constructed from, or lined with a material resistant to corrosion, and containing supports to hold the specimens so that the significant surfaces are at an angle of 15-30 to the vertical and facing upwards. The operating temperature inside the cabinet is 50 + 1 and the test solution is sprayed through nozzles at a rate that the spray collected over a horizontal area of 8000 sq. mm during 8 hours averages 1.5 + 0.5 ml/hr, taking care that no liquid falling from the specimens or parts of the cabinet is collected. Baffles prevent direct impingement of the spray onto the specimens. The air used to provide the spray is humidified by passing through saturation tower co