Chapter 6 - Corrosion

• What is Corrosion???

• Forms of Corrosion

• How to design for Corrosion

• Dr. Payer ASM video tapes – worth the effort

METALS WANT TO CORRODE – they want to exist as oxide compounds because oxides contain less energy and are more stable!!

What is Corrosion??

• Electrochemical reaction involving an anode and a cathode.

• Deterioration of a material because of reaction with the environment.

• Combines many elements of engineering and impacts ALL engineering disciplines: Chemical Engineering, Mechanical Engineering, Material Engineering, Electrical Engineering and Civil Engineering

What is Corrosion??

• Corrosion involves the interaction (reaction) between a metal or alloy and its environment. Corrosion is affected by the properties of both the metal or alloy and the environment. The environmental variables include:

– pH (acidity) – Oxidizing power (potential) – Temperature (heat transfer) – Velocity (fluid flow) – Concentration (solution constituents)

What is Corrosion??

• Cost? EQUALS 3 – 5% of GNP/ year or $700/person based on 2006 estimate = $300 billion US only (corrosion of steel – the biggie)

• Combination of the material and it’s environment - Examples:– No Problem:

• Lead in Water• Aluminum in atmosphere• Nickel in hydraulic fluid

– BAD:• Steel in marine environment• Cu in Ammonia• SS in chloride (Sea water)• Lead in wine

Requirements for Corrosion:

Ionic – Current Path

Electronic Path

ANODE CATHODE

Where Corrosion Occurs!!!!

Fe Fe 2+ + 2e-

2H+ + 2e- H2

Fe + 2H+ Fe 2+ + 2e- =

Anodic partial process (oxidation of iron)

Cathodic partial process (reduction process – H reduced)

• Previous corrosion was Fe in HCL.

• Can also have Fe corrode in water – most common form of corrosion (i.e. steel left outside). – The anodic corrosion reaction is the oxidation

of iron: Fe Fe2+ + 2e-

– The cathodic or reduction reaction is the reduction of oxygen: O2 + 2H2O + 4e- 4OH-

Relationship between the rate of corrosion, corrosivity of an environment and corrosion resistance of a material.

Methods to Control Corrosion

There are five methods to control corrosion:

material selection

coatings

changing the environment

changing the potential

design

Description of Environment in terms of oxidizing power (E) and pH

Key Parameters:•pH – next slide•Oxidizing power =measure of the tendency of a chemical species to acquire electrons and thereby be reduced. Reduction potential is measured in volts (V), or millivolts (mV). Each species has its own intrinsic reduction potential; the more positive the potential, the greater the species' affinity for electrons and tendency to be reduced

•Oxidizing power = measure of relative tendency to corrode or oxidize – a solution of low oxidizing power will corrode only those metals at the lower end (more active) of an emf series.

Figure 1. The scales of pH and pOH.

pH = - log10[H+]

pOH = - log10[OH-]

Pure water has a pH of 7

Strong acids have lower pH’s which means they have more H+ ions!

Strong alkali’s have low pOH’s which means they have more OH- ions!

The more ions, the more “toxic” the solution. But not that simple – go to materials Pourbaix (potential pH diagram)!!

Recall, Corrosion is the degradation of a metal by an electro-chemical reaction.

One half of this is the dissociation reaction of a metal M into a metal ion, Mz+ , releasing electrons e-

M Mz+ + ze-

where z, an integer of 1, 2, or 3, is the valence of the metal.

Acidic environments, with high [H+] (and thus low pH) stimulate this reaction; thus a metal such as copper, in sulphuric acid solution, reacts rapidly

Cu Cu2+ + 2e H2SO4 2H+ + SO4

2-

Some metals are resistant to attack by some acids because the reaction product, here CuSO4 , forms a protective surface layer; thus lead-lined containers are used to process sulfuric acid because lead sulfate is protective.

Most metals are immune to attack by alkalis because their hydroxide, formed in the reaction, is protective. There are, however, exceptions, notably aluminum, that forms non-protective aluminum hydroxide, Al(OH)3.

Cu2+ + SO42- CuSO4

Metals behavior as function of oxidizing power (E) and pH

The “Right” material depends on the environment.

Polarization can have a major effect on metal stability.

Often several approaches to control corrosion

Often several “system” constraints pertain

Eight forms of corrosion can be identified based on appearance of the corroded metal. These are:

• Uniform

• Galvanic, or two-metal

• Pitting

•Crevice or Concentration Cell

• Intergranular

• Stress corrosion cracking

• Erosion-corrosion

• Dealloying

Uniform Corrosion• Most common – i.e. steel exposed to environment.• Uniform in nature – leaves scale or deposit over entire exposed

area – this is called rust which is really iron-oxide – Fe(OH)3 or Fe2O3

• Fairly predictable and therefore the effects can be minimized!– i.e. corrosion proportional to current, proportional to time (corrosion rate)

• < 2 mils/yr – necessary for food containment• 20 mils/yr = conservative estimate for general atmospheric corrosion.

• Really general form of galvanic corrosion – i.e. anode and cathode random and in same material!

• Prevented by– Removing electrolyte (i.e. lower relative humidity below 30%)– Choose material that doesn’t rust in a particular environment – look at

potential-pH diagram!– Add design “allowance” for rust

Uniform (or general) corrosion of steel in water:

Uniform Corrosion

• Corrosion penetration rate (mils/yr):

At

KWCPR

Constant depending on desired units

Weight loss after exposure time t

Exposure time

Exposed area

density

Uniform Corrosion:

• Corrosion rate in terms of current:

nf

ir r = rate in terms of mol/m2-s

i = current per unit surface area of material corroding

N = # of electrons associated with ionization of metal ion

F = constant = 96,500 C/mol

A rate of less than 2 MPY is necessary for food containers

A rate of less than 20 MPY for many industrial applications

METAL: Carbon SteelENVIRONMENT: Industrial enFORM OF CORROSION: GeneralMETHOD TO CONTROL! Surface is painted for protection

EXAMPLE 1:MIG Welding tank

Question: The tank sees tension stress due to internal gas pressure, would this lead to stress corrosion cracking as well as general??

EXAMPLE 3:Machine Shop Table

METAL: Carbon SteelENVIRONMENT: Industrial enFORM OF CORROSION: GeneralMETHOD TO CONTROL! Surface is painted for protection. Aggressive environment (molds dragged acrosssurface) led to scrapping off of paint. Note corrosion where paint is scraped off in line.

EXAMPLE 4:Dumbbell

METAL: Cast IronENVIRONMENT: Indoor (exercise room)FORM OF CORROSION: GeneralMETHOD TO CONTROL! Surface is painted for protection. Note, portion of dumbbell where paint was abraded off due to handling shows significant corrosion while areas that are better protected from abrasion retained paint and therefore show little corrosion.

EXAMPLE 5:House Drain and Drain Cap

METAL: Cast IronENVIRONMENT: Residential basement – water exposureFORM OF CORROSION: GeneralMETHOD TO CONTROL! Surface is painted for protection. Note the 1 year old cap shows significantcorrosion already!

1 year old cap

30 year old cap

60 YEAR OLD OIL PUMP

Kinzua Viaduct – see photos!! (1882/1900)

Crevice or Concentration Cell

• Local attack (corrosion) in crevice due to change in chemistry of electrolyte making it more aggressive – i.e. stagnant fluid = lower oxygen concentration = decrease in pH.

• Can be between metal surfaces or non-metal surfaces in contact with metal.

• Very destructive since highly localized!• How design around?

– Leak proof weld– Better gasket design– Avoid stagnant water

Crevice or Concentration Cell

• Good example – crevices and recesses or under deposits of dirt or corrosion products where the solution is stagnet.– Crevice must be wide enough to allow

solution to penetrate yet narrow enough for stagnancy (i.e. few thousandths of an inch).

Depending on the environment developed in the crevice and the nature of the metal, the crevice corrosion can take a form of:

pitting (i.e., formation of pits), filiform corrosion (this type of crevice corrosion that may occur on an aluminium surface underneath an organic coating), intergrannular attack, or stress corrosion cracking.

Crevice or Concentration Cell

KEY – In crevice there are high concentrations of H+ and Cl- ions which are especially corrosive!

WHY? Low oxygen levels (stagnant) means ions have nothing to react w/ except the metal!!

Crevice corrosion between pipe and I-beam: Rubber pads just accelerated

the attack – why???

EXAMPLE 2:Track Fastener - Taipei

METAL: Ductile Cast Iron per ASTM D412ENVIRONMENT: Corrosive Salt Water (Salt Spray)FORM OF CORROSION: Crevice Corrosion + GeneralMETHOD TO CONTROL! Surface is degreased, sand blasted and phosphatized for corrosion protectionSurface is then painted with Chemlock elastomer primer and bonding adhesive.

Note: Salt water trapped between elastomer and steel led to crevice corrosion which led to underbond corrosion. The adhesive to metal bond then failed causing the elastomer to delaminate. Resulted in returnof several million dollar worth of product + replacement costs (labor and components).

A manufacturer’s nightmare!!

Pitting Corrosion:• Extremely localized corrosion that leads to the creation

of small holes in the metal surfaces • The driving power again is the lack of oxygen around a

small area. This area becomes anodic while the area with excess of oxygen becomes cathodic.

• More of a problem in stagnant solutions.• Very destructive since highly localized.• Prevention?

– Material selection– Avoid stagnant flow

Pitting Corrosion:• Similar in chemistry to crevice corrosion except it

happens in pits.• Occurs in “pits” of metal surfaces where again,

electrolyte is aggressive (stagnant).• More of a problem in stagnant solutions.• Very destructive since highly localized – may go

undetected until failure occurs.• Gravity causes pit to grow downward – corrosion

rate can increase with time

Pitting Corrosion:• A pit can be initiated by a localized surface

defect, scratch or slight variation in composition.

• Stainless steels are especially susceptable to this form of corrosion.

• Prevention?– Material selection– Avoid stagnant flow– Alloy SS with about 2% molybdenum.