Water Corrision Mechanisms

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M ay 19 99 P R A C T I C A L G U I D E 5 7

PRACTICAL GUIDE

w a t e r t r e a t m e n t

Water Corrosion MechanismsBy Paul R. Puckorius

his article addresses the cause and con-

trol of corrosion of metals commonly

found in cooling water systems. The

pipes, tubes, coils, water boxes, and cool-

ing tower must be protected from corrosion.Each cooling water system has a unique combi-

nation of metals, water flow, temperatures, and

makeup water quality that must be considered

before selecting a corrosion control program.

The first step in selecting this treatment is to

identifyallmetals in the system. Then, we must

know the limitations of the corrosion inhibitors.

Finally, cost- effective corrosion inhibitors can

be selected. However, first we need to under-stand corrosion.

In the corrosion cell illustrated inFigure 1, a neutraliron atom loses two electrons and becomes an iron ion,Fe++, in solution in the electrolyte. The electrons flowthrough the metallic path to the cathode and back intothe electrolyte. They combine with two positively chargedhydrogen ions in the electrolyte. The resulting molecule

of hydrogen escapes as a gas.If this highly simplified diagram told the entire story,the system would reach equilibrium and corrosion wouldstop. This condition is called passivation. However, ad-ditional reactions take place. The removal of hydrogenions from the electrolyte at the cathode (producing hy-drogen molecules) leaves an excess of hydroxyl (OH)ions. These hydroxyl ions react with the iron ions in thesolution forming ferrous hydroxide.

Fe++ + 2 OH Fe(OH)2

The ferrous hydroxide Fe(OH)2dissolves, allowing the

corrosion reaction to continue.In oxygenated water, as in open systems like cooling

towers, further reactions occur. Dissolved oxygen reactswith ferrous hydroxide at the anode to produce ferrichydroxide, which is less soluble than ferrous hydroxide.At the cathode, dissolved hydrogen reacts with dissolvedoxygen to form water. A more comprehensive picture ofgalvanic corrosion in the presence of oxygen is the sameasFigure 1 with these reactions:

Fe++ + 2 OH Fe(OH)2

4 Fe(OH)2+ O

2+ 2 H

2O 4 Fe(OH)

3

O2

+ 2 H2O + 4 e 4 OH

4 H+ + 4 OH- 4 H2O

Figure 2 illustrates this phenomenon.

Galvanic CorrosionThe galvanic series (Table 1) is a table that allows engi-

Paul R. Puckorius, president of Puckorius & Associates,Inc., has more than 35 years experience in water treat-ment and troubleshooting. His company is an indepen-dent water and wastewater management consulting firmthat provides technical consulting and does not sell chemi-cals or equipment.

T

Electrical Nature of CorrosionCorrosion is the dissolution of materials of construc-

tion by their environment. It includes rusting of iron,rotting of wood, weathering of stone, bearing wear, andeven the vaporizing of a satellite re-entering the atmo-sphere. In this article, we consider only aqueous corro-sion of metal. This is always associated with an electro-chemical reaction, as shown inFigure 1.

Electricity flows between metal areas through a solu-tion. Corrosion takes place where electrons leave the

metal and ions enter the solution. This area is called theanode. The area where electrons return to the metal iscalled the cathode. Besides the conducting solution(electrolyte) and the two electrodes (anode and cath-ode), there must be an electron path to complete theelectric circuit. The metal structure itself may providethe electron path, or the circuit may be completed byphysical contact between the metals. The simplest ex-ample is galvanic corrosion of two dissimilar metals. Fa-miliar examples are steel pipe screwed into a copperfitting and mild steel tubes in a brass tube sheet of a heatexchanger.

The following article was published in ASHRAE Journal, May 1999. Copyright 1999American Society of Heating, Refrigerating and Air-

Conditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper

form without permission of ASHRAE.


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Water Treatment

neers to predict which metal will corrode when differ-ent metals are in contact. In most cases, the metal that ishigher in the table will corrode, and the metal that islower in the table will be protected.

The galvanic series predicts only whether corrosion canoccur. Corrosion rates are determined by the following:What metals are electrically coupled. The relative anodic to cathodic surface areas. A larger cathodeA higher corrosion rate. A smaller cathodeA lower corrosion rate. A large anodeGeneral sheet type corrosion.A small anodePitting type corrosive attack.It is not always necessary to have two dissimilar metals

in the system. Many metals, particularly iron, can haveanodic and cathodic areas at the same time. The poten-tial differences between anodic and cathodic areas inthe same metal are the result of grain boundaries, grain

orientation, differential grain size, differential thermaltreatment, surface roughness, scratches, differential strain,etc. When immersed in an electrolyte, current flows fromthe anodic to the cathodic areas and corrosion results.The anode and cathode are the same as for the galvaniccell. The metal itself forms the electron path. Where thecorrosion in the galvanic cells in a single metal is moreor less uniform, this action tends to cause pitting.

Even a completely homogenous metal can corrode ifit is immersed in a nonhomogenous liquid. The classicalconcept of a concentration cell is shown inFigure 3. Ingeneral, the specimen in the more concentrated solu-

tion is anodic to the specimen in the more dilute solu-tion. A common type of concentration cell occurs in crev-ices or dead areas such as those under deposits. Thisphenomenon is known as crevice corrosion and is illus-trated inFigure 4.

The crevice above, for example, hinders the diffusionof oxygen into the wetted area of the overlap of tworiveted members. The result is high oxygen in the bodyof water and low oxygen in the crevice, causing a con-centration cell. If the metal involved is steel, the area oflow concentration is anodic; if copper, it is cathodic.

Because an electrochemical cell is necessary for corro-sion, the logical method of preventing corrosion is to de-stroy the cell. One method of doing this is to impose anonconducting barrier between the metal and theelectrolyte.

Corrosion Control ConceptsThe rate of attack by circulating cooling water on steel

is a function of temperature and pH. It increases withtemperature and depression of pH.

Corrosion protection follows two rules:1. High pH promotes scale and inhibits corrosion.2. Low pH inhibits scale and promotes corrosion.

Figure 5 illustrates the effect of pH on the corrosionrate of unprotected mild steel in water.

Corrosion control relies on a barrier between the metaland the corrosive medium. Barriers can be calcium car-bonate or calcium phosphate scale that forms within thesystem. These relatively thick barriers have the seriousdrawback of impeding heat transfer. On the other hand,chemical inhibitors can be used to form a thin protectivefilm. The film can be metallic or non-metallic.

Steel surfaces have both anodic and cathodic areas in aratio of about 80:20. The traditional view is that the ferritephase acts as the anode and the cementite phase acts as the

Figure 1: Typical corrosion cell.

Figure 2: Electrochemical reaction.

Figure 3: Differential corrosion.

Figure 4: Crevice corrosion.


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Practical Guide

cathode. However, the situation is seldom so simple.If the anode could be isolated from the electrolyte by

some sort of film, corrosion could be controlled. In 1899,engineers found that certain oxidizing materials, such asnitrites, would inhibit corrosion. This inhibition is the re-sult of an adherent, insoluble oxide film, 30 to 50 A thick,that forms on the anode surface by a reaction between theiron going into solution, the oxidizing inhibitor, and/oroxygen in solution. There has been much controversy asto the composition of these films, so they will be referredto as gamma iron oxide films (GFe

2O

3or mayhenite).

Most oxidizing agents, including oxygen, are anodicinhibitors. Steel will not corrode in salt water coveredwith oxygen at several atmospheres pressure. Anodic in-hibitors oxidize the anodic base metal and form a thin,tightly adherent, oxide layer that interrupts the electri-cal circuit.

The most efficient anodic inhibitor is the pertechnateion. Unfortunately, for the users of circulating water, it ishighly radioactive.

The most popular anodic inhibitor was the chromateion. It is inexpensive, very soluble, and quite easy touse. Although chromate is no longer used because ofenvironmental concerns, it is the model anodic inhibi-tor. Other anodic inhibitors include nitrates, molybdates,tungstates, orthophosphates, silicates, ferricyanides,persulfates, borates, and benzoates. To be effective, highconcentrations of these oxidizing inhibitors are required.If the chromate concentration dropped much below 500

ppm, general corrosion was controlled, but severe pit-ting attack could occur.

Assume the system shown inFigure 6is corroding atthe rate of 10 mils per year. Now add enough anodicinhibitor (about 50 ppm chromate) to convert 90% ofthe anodic surface to iron oxide, as shown schematicallyinFigure 7. The current flow remains essentially the same;the amount of iron removed remains essentially the same,but iron is removed from only 10% of the anode. There-fore, the corrosion rate on this reduced area is 100 milsper year, a severe pitting condition.

Pitting is usually more severe on the cool end of an

exchanger. A rule of thumb states that the rate of a chemi-cal reaction doubles for every 10F increase in tempera-ture. Thus, the iron oxide film would form faster, tighter,and more completely on the hot end of the exchanger.

To prevent pitting, a protective film should be depos-ited in the cathode areas as well as in the anode areas.

In the early 1940s, engineers found that a mixture ofsodium polyphosphate and sodium nitrite had greater in-hibitive properties than would have been predicted by thesum of their individual inhibitive effects. Lacking a betterterm, such mixtures were called synergistic inhibitors.Today, the preferred term is cathanodic inhibitors.

Referring back toFigure 6,we see that simultaneouslywith the oxidation reaction taking place at the anode, areducing reaction is taking place at the cathode, produc-ing hydroxyl ions. Thus, the cathodic surface is coveredwith a thin layer of hydroxyl ions, giving it a pH muchgreater than that of the body of the water. It has beencalculated that this film has a thickness in the 300 to 500nanometer (1 nanometer = 109 meters) range along witha pH of approximately 11.0 to 11.5 when the bulk waterpH is 8.0.

Consider natural water circulating through the system.Natural waters contain calcium ions plus bicarbonate and/

Figure 5: Effect of pH on corrosion rate. Figure 6: Unprotected sur face corrodes uniformly.

Figure 7: With 90% of the surface protected, the 10% unprotected

corrodes at 10 times the average rate.

RelativeCorro

sionRate

pH


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Water Treatment

or carbonate ions. Even though calcium carbonate is con-sidered an inefficient cathodic inhibitor, it is well suited todemonstrate the mechanism of the cathodic inhibition re-action. Calcium and carbonate ions in the water migrateinto the high pH film, precipitate, and form a layer of cal-

cium carbonate thick enough to effectively stop the elec-tron transfer (or current flow) but thin enough that it doesnot decrease heat transfer. For this process to be success-ful, any particle of calcium carbonate that extends beyondthe high pH zone must redissolve.

Perhaps the best cathodic inhibitor is polyphosphate.It reacts in the high pH zone much the same as doescarbonate, forming a calcium polyphosphate film. Thedifficulty is that some polyphosphate reverts to ortho-phosphate, which also reacts with calcium, in the highpH zone, forming scale rather than a film. For this rea-son, it is necessary to keep the pH or the phosphateconcentration quite low (pH = 6.2 to 6.7 with 30 to 50

ppm PO43

).In the last few years, a series of calcium phosphatestabilizers, such as the hydroxylated acrylate polymers,has been developed. These permit higher concentrationsof calcium and phosphate ions to be carried in the circu-lating water, again increasing the probability that theywill migrate into the high pH zone.

Film Formation versus Film Maintenance All corrosion inhibitors require a protective film to

form initially and then to maintain this film with the in-hibitors (see Table 2 ). If this is not done properly, theinitial levels of inhibitor will not establish good corro-sion control. Actually, levels that are too low could ac-celerate corrosion.

When various inhibitors are used, some establish pro-tection in a short time even at low levels. Others requirehigher levels for longer periods. Chromate takes two tothree days, phosphates three to five days, zinc five to sevendays, and polysilicate and molybdate up to two weeks.

Changes in TechnologyThe last several years have seen major changes in both

water treatment technology and water system design/operation.

New galvanized cooling towers are seeing faster form-ing and more extensive corrosion of the galvanized coat-

ing (white rust) compared to older galvanized towers. Acontinuing argument exists between galvanized towermanufacturers and water treatment suppliers as to whois to blame. Each has some responsibility. The water treat-ment can be a contributing factor, while some towermanufacturers use mill galvanized sheet, which has a thin-ner coating and uses a different alloy than the older hotdip galvanizing process. Even the passivation of the gal-vanizing has changed to a less effective process. Yet ifnew galvanized cooling towers are properly passivatedupon commissioning and the water treatment programincorporates the proper inhibitors, white rust can be

minimized or even prevented.Chillers are becoming more efficient through the use

of better heat transfer between the cooling water andthe refrigerant. Internally smooth copper tubes have foryears been standard condenser design for cooling water.The refrigerant has seen externally finned copper tubesfor many years. Today, new chillers often have copper

Table 1: Galvanic series of common metals found in cooling

water systems.


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Practical Guide

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Table 3: Recent cooling water treatment changes.

condenser tubes that are rifled internally, similar to riflebarrels. They are referred to as enhanced tubes andincrease heat transfer from the cooling water.

There are even super enhanced tubes that have lowprofile fins on the internal surfaces. These new enhanced

copper tubes are only about half as thick as smooth tubesunless thicker tubes are specially ordered. The new tubeshave greater stressed areas that can result in increasedcorrosion and early perforation. Deposits can accumu-late in the grooves resulting in fouling that reducesheat transfer and can lead to under deposit corrosion.Conscientious water treatment can minimize these prob-lems but cannot prevent failure or loss in heat transfer.The balance between increased chiller efficiency and com-plex water treatment presents a design dilemma and achallenge for operators.

Besides changes in materials of construction, watertreatment programs have changed. Table 3 compares

where we were 10 to 15 years ago to where we are nowwith conventional cooling water treatment programs.Chromate is an outstanding corrosion inhibitor for mild

steel, copper alloys, galvanized steel, and aluminum. Itgives many years of equipment life and is rapid toreprotect if protection is lost. It is forgiving in that itrecovers quickly from upset conditions. It not only is notfood (nutrient) for microorganisms, it is somewhat toxicto them. It works over a wide pH range of 6.0 to 10.0,does not form scale or deposits, and colors the wateryellow, so we can detect its presence by just looking atthe water. It was almost a perfect water corrosion inhibi-tor. However, the EPA considers it toxic, a potential car-cinogen, and a potential air and water pollutant. TheEPA banned chromate for HVAC cooling water systems in1995 and in industrial systems several years later.

With chromates no longer available, the water treat-ment industry turned to a variety of blends of corrosioninhibitors, first zinc salts, then molybdate salts, and mostcommonly now phosphate-based chemicals for mild steelcorrosion control often along with nitrogen-based chemi-cals for copper alloy corrosion control. Phosphate andnitrogen chemicals are nutrients for microorganisms, al-gae, weeds, and grasses (they are fertilizer). Not only dothese chemicals provide less corrosion protection thanchromates, they also provide food for microbiologicalorganisms.

At the same time, to improve corrosion protection ofcooling system metals, the pH was raised from less than7.5, with chromate-based corrosion inhibitors to pH lev-els above 7.5 often up to 8.5 to 9.5. This change hadboth good and bad effects. The good effect is that it re-duces or eliminates the need to use acid to keep pHlevels down. Increased safety of personnel and reduceddamage due to overfeed were welcome changes.

However, higher pH levels have a number of disad-vantages:

Higher pH (particularly above 9.0) increases copperand galvanized steel corrosion.

Higher pH (particularly with hard water and withphosphate treatment) requires high levels of scaleinhibitor with resulting substantially increased costsfor cooling water treatment.

Higher pH reduces the effectiveness of many bio-cides, including chlorine, thus restricting the avail-ability of effective biocides, usually increasing bio-control costs, and making biological control moredifficult.

Some scale inhibitors used when the pH is higherare corrosive to copper alloys, galvanized steel, andeven mild steel, particularly if overfed. These scaleinhibitors also could contribute to scale, so if a littleworks, a lot may be harmful.

Conclusion

Understanding both corrosion mechanisms and newtechnologies of HVAC cooling system design and watertreatment are essential to maintaining efficient opera-tion and protecting all components in the system. Engi-neers and operators should not assume but should findout what new equipment and technologies may exist andwhat action is necessary to provide sound operation. Newwater treatment technologies may be necessary, but us-ers should obtain valid verification of their performancebefore using them and then monitor the results. If re-sults are not as expected, make changes before your sys-tem deteriorates.

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Water Corrision Mechanisms