Module 2 “The Forms of Corrosion”

Corrosion Forms on Metals

• General

• Localized (pitting and crevice)

• Galvanic-

• Stress Corrosion Cracking

• Flow-assisted

• Intergranular

• Dealloying

• Fretting

• Corrosion Fatigue

• High-temperature

General Corrosion Definition

• Characterized by chemical reactions, oxidation and reduction, that proceed uniformly over the entire wetted surface.

• Uniform attack, general corrosion, represents the greatest destruction of metal on a tonnage basis.

• This form of corrosion is easily monitored and the life of equipment accurately estimated.

General Corrosion

General Corrosion due to Dissimilar Surfaces

• Mill Scale = Cathode

• New (Anode) –Old Pipe (Cathode)

• Stressed Surfaces= Anode

• Welded Sections (Heat Affected Zone)= Anode

New—Old Pipe Cell

Old Pipe (cathod

e)

New Pipe

(anode)

Old Pipe (cathod

e)

Different Stress

The portion of the metal under higher stress, (eg. cold worked) will be anodic to the portion of the metal at a lower stress level.

High Stress =Anode

Low Stress = Cathode

General Corrosion due to Dissimilar Electrolytes

In most cases, the metal immersed in the more conductive solution will be the anode, and the metal immersed in the less conductive solution will be the cathode.

More Conductive = AnodeLess Conductive = Cathode

General Corrosion due to Oxygen Concentration

The metal immersed in the oxygen lean electrolyte will be the anode, and the metal immersed in the oxygen rich electrolyte will be the cathode.

Oxygen lean = Anode

Oxygen rich = Cathode

Metal Ion Concentration

• Cathode–in saturated solution

• Anode –in unsaturated solution

General Corrosion Remedies

• Coatings

• Linings

• Alloy selection

• Change the chemistry

• Corrosion inhibitors

• Cathodic or anodic protection

General Corrosion Spray Tower

• Severe corrosion of the inner carbon steel walls at the mid-section of a tower between the two spray levels was found.

• More than half of thickness was gone from the original 1/4 plate in 8 years.

• Notice support beams from upper level to the lower level.

• Cause high temperature and condensation water from sprayed detergent slurry.

Neutralization Mixers

Reactors that see various concentrations of sulfuric acid should use Alloy 20 UNS No- N08020.

Heating Coils

• Heat transfer surfaces are very susceptible to general and localized pitting corrosion due to the higher temperature.

• Material selection for heat transfer surfaces is therefore important.

AtmosphericAtmospheric Corrosion Example

• Electrical Panels are an area where corrosion can play havoc. For example where the surrounding air is high in salt or acidity.

• Electrical firesElectrical fires have been associated with corroded contacts.

• Sealing these areas from the outside air and moisture is important.

Handouts Reference General Corrosion

Carbon Steel- pp. 373-403 “Carbon and Low Alloy Steels”

Stainless Steel- pp. 553-559 “Corrosion of Stainless Steels”

Localized Corrosion Definitions• Pitting

A deep, narrow corrosive attack in a localized region which often causes rapid penetration of the substrate thickness.

• Crevice Corrosion

A form of localized attack in which the site of the attack is an area where free access to the surrounding environment is restricted..

• Filiform Corrosion

A special form of oxygen cell corrosion occurring beneath organic or metallic coatings on materials. The attack results in a fine network of random “threads” of corrosion product developed beneath the coating material.

Pitting on Stainless Steel 304 SS

• Micro view of pitting at 50x magnification

Copper Pitting

Pitting Remedies

• Use more alloyed material ( molybdenum is key alloying element)

• Reduce eliminate the pitting agents

• Lower the process temperature

• Coatings

• Cathodic protection

• In the case of SS increase process velocity

Electrochemical Lab Pitting Data

• First graph showing the effect of pH and Cl- at 35C

• Second graph showing the effect of increasing temp. with 4 diff. Cl- conc.

• The third graph showing pitting potentials with different alloys with increasing temperatures.

Heat Exchangers(above 140F with salt)

• 316L Stainless Steel dryer heat exchangers are susceptible to localized pitting of heat exchanger tubes . Past inspections showed shallow pits on the inner tube surfaces that eventually led to failure.

• Dryer heat exchangers should be fabricated out of a material with at least 4% molybdenum to avoid pitting. 316 has only about 2% molybdenum.

Handouts Reference Pitting

• Carbon Steel p. 385,389 “ Carbon and Low Alloy Steels”

• Stainless Steel p.113-114 “Localized Corrosion”

Exercise 2 Soft-soap Business Issue”

Instructions:

• Form into groups of three with people from other functional groups if possible.

• Look at slide pictures

• Read Background

• Answer Questions

• Be prepared to share your answers with the rest of the class

Background “Soft-soap Pump Springs”• A Soft-soap product

experienced corrosion of 302 SS pump springs that could be seen by the consumer on the store shelf

• The corrosion occurred mainly on spring parts that were in contact with the plastic. Shown here is corrosion on last two winds

Crevice Corrosion Background

M+Cl- + H2O = MOH + H+Cl-

Corrosion Inside Crevice (Anode)

Chemical Reaction with Salt NaCl

Electron Micrograph of 302 Spring

Showing corrosion also occurring between spring coils

Results of Statistical Analysis

• Conditions: 120F in Pump after 49 days

Results of Statistical Analysis

Another Example of Crevice Another Example of Crevice CorrosionCorrosion

Exercise 2-1 Soft-Soap Pump Springs

• What type of corrosion occurred on the 302 SS spring?

• What would be two possible solution based on the statistical data and remedies?

Questions to answer

Crevice Corrosion Remedies• Use continuous complete penetration welds

• Laps joints should be seal welded

• Remove deposits frequently

• Use proper packing materials in joints

• Avoid crevices in heat exchanger tube sheet

• Use higher alloyed material or plastic

• Use a corrosion inhibitor such as Sodium sulfate

• Alter the chemistry by adjusting the pH or lowering the salt concentration

Forms of Cracking Corrosion Definitions

• Stress Corrosion CrackingRefers to metal cracking caused by the simultaneous presence of tensile stress and a corrosive chemistry.

• Corrosion FatigueResults from the combined action of a cyclic tensile stress and and a corrosive environment.

• Hydrogen-Induced Cracking (HIC) Results from the combined action of a tensile stress and hydrogen in metal.

Stress Corrosion Cracking Susceptibility

Chloride Stress Cracking Temperature

Above 140 F Stress Corrosion Cracking occurs. Therefore if possible avoid continuous process operations above this temperature with 304/316 SS

Cracking - On A Microscopic Level

• Cracks that run across grains is called transgranular

• Cracks that follow grain boundaries are called intergranular

Stainless vs. Other Alloys Stress CrackingStainless vs. Other Alloys Stress Cracking

Lab Data

Lye Separators

• After about 9 years stress corrosionstress corrosion cracking was pervasive in a continuos lye seperation plant. A typical crack is shown here.

• The material of construction was 304 SS. Crack

Stress Corrosion Cracking 100x mag.

Area AArea A

Area BArea B

Area CArea C

Carbon Carbon steelsteel

Carbon steel Carbon steel stress relievestress relieve

Nickel Nickel based based alloysalloys

Carbon Steel Caustic Soda Service ChartCarbon Steel Caustic Soda Service Chart

Sodium hydroxide Sodium hydroxide concentrationconcentration

00 100100

180 F180 F

Stress cracking Stress cracking lineline

120 F 120 F

Ammonium Chloride

This is an example where a tank fluid is not compatible with 316 stainless steel. Pitting and stress corrosion cracking with quaternary ammonium chloride fabric softener.

Stress Cracking Remedies• Evaluate a Duplex alloy- 2205 or 2507

• Increase nickel concentration in the alloy

• Lower hardness of alloy through heat treat

• Lower process temperature (60C max)

• Cathodic Protection

• Remove the bad actor ions such as chloride

Lye Evaporators

During the evaporation step, lyes can be corrosive to 316 SS alloy heat exchangers by pitting and stress corrosion cracking. In addition lyes can contain NaClO3. NaClO3, Sodium Chlorate, is corrosive to stainless steels. If this is the case, an upgrade in heat exchanger metallurgy is necessary to Inconel 625 for tubing.

Fatigue Cracking

Expansion joints that undergo high stress and strain due to heat/mechanical forces are susceptible to corrosion fatigue corrosion fatigue crackingcracking. In general the higher the nickel concentration in the alloy the better the fatigue properties.

Corrosion Fatigue Remedies

• Reduce the grain size

• Improve the surface finish

• Increase alloying elements ( Ni, Cr, Mo)

Surface Preparation and Corrosion Fatigue

• In general the fatigue life increases as the magnitude of surface roughness decreases.

• Best- A Metallographic finish, free of machining grooves and grinding scratches

Hydrogen Induced Cracking

Galvanic Corrosion Definition

Corrosion due to the electrochemical potential difference between different metals. However, potential differences can be caused by differences in the homogeneity of the same metal.

Corrosion Galvanic

More Reactive

Less Reactive

PotassiumMagnesiumBerylliumAluminumZincChromiumIronNickelTinCopperSilverPlatinumGold

Galvanic Series

Galvanic Example in Bottle Making• Equipment that is used to make plastic bottles often

times uses different metals to conduct heat at different rates. For example beryllium copper and aluminum. This alloy combination allows bottle thicknesses to differ for design purposes.

• Galvanic corrosion can and did occur at these joints. Replacement of the expensive parts were needed.

• In this case the water for cooling needs to be buffered to a pH of 8 and eliminate as much salt as possible.

Galvanic Corrosion Remedies

• Choose alloys of similar chemical potential

• Avoid bad area ratios: large cathode/small anode surface areas

• Insulate dissimilar alloys

• Coatings apply with caution ( pinhole free)

• Change the environment ( chemistry)

Flow-Assisted Corrosion Definitions

• Erosion-corrosionOccurs when the velocity of the fluid is sufficient to remove protective films from the metal surface.

• Impingement CorrosionLocalized erosion-corrosion caused by turbulence or impinging flow.

• Cavitation CorrosionMechanical damage process caused by collapsing bubbles in a flowing liquid.

Impingement- Erosion

Plodder Worms- Mixers

Typically are silicon aluminum castings . These alloys are susceptible to erosion and pitting erosion and pitting corrosioncorrosion with time.

Erosion Corrosion Remedies

• Change the metallurgy ( adding chrome and moly 316 SS)

• Alter the design ( provide for large sweeps, eliminate 90 elbows)

• Lower the velocity ( less than 2 meters per second)

• Raise the pH ( pH above 9 helps)

• Lower the temperature

• Deaerate

• Filter

• Weld overlays or metallic coatings

• Rubber linings

Intergranular Corrosion Definition

• Corrosion due to the preferential attack at, or adjacent to, the grain boundaries of a metal.

• Most often associated with welds in low pH fluids.

• Caused by chromium being tied up by carbon.

Intergranular Corrosion 500 x mag.

Dark areas are corrodedDark areas are corroded

Intergranular Corrosion Remedies

• Use low carbon stainless steel (316L)

• Ti Stabilized steel in Europe

• Use higher alloy filler metals

• Note: Nitric acid can cause intergranular corrosion of sensitized (welded) 304 and 316 SS.

Fretting Corrosion Definition

• Fretting corrosion is defined as metal deterioration caused by repetitive slip at the interface between two surfaces in contact.

• It is not corrosion due to rotation or erosion.

• More exactly.. motion from vibration and corrosion effects during that time.

Fretting Corrosion Examples

• Axles

• Housings and shafts

• Spline connections

• Pin joints

• Riveted lap joints

Resistance to Fretting CorrosionUnder dry conditions

Steel on steel Low

Nickel on steel Low

Aluminum on Aluminum Low

Zinc on steel Medium

Copper on steel Medium

Silver plate on steel High

High-temperature Corrosion

• A form of material degradation classified above ( 750+ F).

• General or pitting corrosion occurs.

High Temperature Corrosion

• Oxidation reactions include water vapor, hydrogen, hydrogen sulfide, ammonia, sulfur dioxide, chlorine, sulfur, carbon dioxide and oxygen

• Development of scales

• Corrosion products

- solids

- liquids

- gases

High Temperature Remedies

• Determine possible mechanism of corrosion

- sulfidation, carburization, hydrogen effect

• Select alloy most resistant to above

• Carbon steel maximum temperature range is 800- 850 F in air.

Summary of Scaling Temperatures in Air

• 1010 Steel ... 900F

• Alloy Steel, 9Cr, 1.0 Mo...1200 F

• 410 SS, 12 Cr ...1400 F

• 304, 321, 347 18 Cr, 8 Ni... 1650 F

• 310 SS, 25 Cr, 20 Ni... 2100 F

Exercise 2-2 Sulfur Burner Exhaust Piping

• High temperature corrosion of 321 SS at 1300 F with SO2

• The thickness of the plate was 1/8 inch and lasted less than two years.

Handout on High Temperature Corrosion

See Corrosion by “Hot Gases and Molten Compounds”

-- Specific to this case pp. 243-248 .

Corrosion of Stainless Steel in Sulfur vapor 1300 F Lab Data

314 16.9 mpy mils per year

310 18.9

309 22.3

304 27.0

302B 29.8

316 31.1

321 54.8

Haynes alloy 556 6

Exercise 2-2 Sulfur Burner Exhaust

• Review the previous data and the High Temperature Corrosion handout.

• What is the term used to describe this type of corrosion attack?

• What should be the recommended material of construction?

What did we learn?• The most common forms of corrosion attack on metals.

• Metals and non-metals can become brittle and crack when misapplied.

• Carbon steel is attacked in a general fashion, making it easy to monitor and correct.

• Stainless steels on the other hand, are attacked in a localized fashion, making it difficult to monitor and correct.

• The critical operating temperature for stress corrosion is above 140 Fahrenheit for stainless steels 304 and 316.

• Stainless steels need to be selected carefully.

• There are different types of high temperature corrosion.

• That corrosion issues can be dealt with in different ways.

• Engineering processes and structures need to be inspected and protected from corrosion.