The Forms of Corrosion-Part2.ppt

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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(cathode)

New Pipe(anode)

Old Pipe(cathode)



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 = Anode

Less 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 thetwo spray levels was found.

• More than half of thickness wasgone from the original 1/4 platein 8 years.

• Notice support beams fromupper level to the lower level.

• Cause high temperature andcondensation water fromsprayed 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 tothe higher temperature.

• Material selection for heat

transfer surfaces is

therefore important.



Atmospheric 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 fires have been

associated with corroded

contacts.

• Sealing these areas from the

outside air and moisture isimportant.



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 oftencauses 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 coatingmaterial.



Pitting on Stainless Steel 304 SS

• Micro view of pitting

at 50x magnification



Copper Pitting



Pitting Remedies

• Use more alloyed material ( molybdenum iskey 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 Dat

• 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 aresusceptible 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

B k d “S ft P S i ”



Background “Soft-soap Pump Springs”

• A Soft-soap product

experienced corrosion of 302

SS pump springs that could beseen 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



pCorrosion



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 loweringthe salt concentration

Forms of Cracking Corrosion Definitions



Forms of Cracking Corrosion Definitions

• Stress Corrosion Cracking

Refers to metal cracking caused by the simultaneous presence of tensilestress and a corrosive chemistry.

• Corrosion Fatigue

Results 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



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 Cracking



Lab

Data



Lye Separators

• After about 9 yearsstress corrosioncracking was pervasivein a continuos lyeseperation plant. Atypical crack is shownhere.

• The material of construction was 304

SS. Crack

Stress Corrosion



Cracking 100x mag.

Carbon Steel Caustic Soda Service Chart



Area A

Area B

Area C

Carbon steel

Carbon steelstress relieve

Nickel basedalloys

Carbon Steel Caustic Soda Service Chart

Sodium hydroxide concentration 0 100

180 F

Stress cracking line

120 F

A i Chl id



Ammonium Chloride

This is an example

where a tank fluid isnot compatible with

316 stainless steel.

Pitting and stress

corrosion cracking with quaternary

ammonium chloride

fabric softener.

St C ki R di



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

L E t



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.

F ti C ki



Fatigue Cracking

Expansion joints thatundergo high stress

and strain due to

heat/mechanical forces

are susceptible to

corrosion fatiguecracking. In general

the higher the nickel

concentration in the

alloy the better the

fatigue properties.

Corrosion Fatigue Remedies



Corrosion Fatigue Remedies

• Reduce the grain size

• Improve the surface finish

• Increase alloying elements ( Ni, Cr, Mo)

Surface Preparation and Corrosion Fatigue



Surface Preparation and Corrosion Fatigue

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

• Best- A Metallographic finish, free of

machining grooves and grinding scratches

Hydrogen Induced Cracking



Hydrogen Induced Cracking

Galvanic Corrosion Definition



Galvanic Corrosion Definition

Corrosion due to the electrochemical

potential difference between different

metals. However, potential differencescan be caused by differences in the

homogeneity of the same metal.



Corrosion Galvanic

Galvanic Series



More

Reactive

LessReactive

PotassiumMagnesium

BerylliumAluminumZincChromiumIron

NickelTinCopperSilverPlatinum

Gold

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 andaluminum. 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 wereneeded.

• 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



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



Flow-Assisted Corrosion Definitions

• Erosion-corrosionOccurs when the velocity of the fluid is sufficient to remove protective

films from the metal surface.

• Impingement Corrosion

Localized erosion-corrosion caused by turbulence or impinging flow.

• Cavitation Corrosion

Mechanical damage process caused by collapsing bubbles in a flowing

liquid.

Impingement- Erosion



Plodder Worms- Mixers



Typically are silicon

aluminum castings .

These alloys are

susceptible toerosion and pitting

corrosion with time.

Erosion Corrosion Remedies



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



Intergranular Corrosion Definition

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

• Most often associated with welds in lowpH fluids.

• Caused by chromium being tied up bycarbon.

Intergranular Corrosion 500 x mag.



Dark areas are corroded



Fretting Corrosion Definition



Fretting Corrosion Definition

• Fretting corrosion is defined as metaldeterioration caused by repetitive slip atthe interface between two surfaces incontact.

• It is not corrosion due to rotation or erosion.

• More exactly.. motion from vibration andcorrosion effects during that time.

Fretting Corrosion Examples



g p

• Axles

• Housings and shafts

• Spline connections

• Pin joints

• Riveted lap joints

Resistance to Fretting Corrosion

Under dry conditions



Under 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



g p

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

• General or pitting corrosion occurs.

High Temperature Corrosion



g p

• 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



g p

• 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



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 Pipin



• High temperature

corrosion of 321 SS at

1300 F with SO2

• The thickness of theplate was 1/8 inch and

lasted less than two

years.

Handout on High Temperature

Corrosion



Corrosion

See Corrosion by “Hot Gases and MoltenCompounds”

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

Corrosion of Stainless Steel in Sulfur



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 27.0302B 29.8

316 31.1

321 54.8

Haynes alloy 556 6

Exercise 2-2 Sulfur Burner Exhaust



• Review the previous data and the HighTemperature 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



• 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 andcorrect.

• 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 140Fahrenheit 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 protectedfrom corrosion.

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The Forms of Corrosion-Part2.ppt