Lecture 3

Types of Corrosion

The Eight Forms of Corrosion

1. Uniform attack (general corrosion)2. Galvanic corrosion3. Crevice corrosion4. Pitting5. Intergraular attack (“IGA”)6. Selective leaching7. Flow-Accelerated Corrosion8. Stress corrosion cracking (“SCC”)

UNIFORM ATTACK or GENERAL CORROSION

• This is the most common form of corrosion.• A chemical reaction (or electrochemical reaction)

occurs over entire exposed surface (or large areas) more or less uniformly.

• Metal thins … fails. • Not usually serious and is typically predictable from

simple tests (e.g., coupon or specimen immersion)• Can be designed “around” by specifying an adequate

CORROSION ALLOWANCE for the expected lifetime of the component.

• Uniform attack minimized by: – specifying proper materials– correctly applying coatings– using corrosion inhibition– protecting cathodically.

•1800-year-old Roman nail shows how iron and steel can withstand burial underground.

•Note: Environment is crucial!

ATMOSPHERIC CORROSION

• Usually “uniform”.• Dry, damp or wet conditions have profound effect on

corrosion.• Dry atmospheres:

– at ambient temperatures, most metals corrode very slowly

– atmospheric oxygen promotes a protective oxide film ... such films are defect-free (sort of!), non-porous (more or less!) and self-healing

– “passivity” of metals like SS, Ti, Cr depends on protective oxide films (but such passivity extends to other environments, e.g., aqueous).

• EXAMPLE:– Ag & Cu tarnish in dry air with traces of H2S

(undesirable - aesthetically, technically - affects electrical contacts, etc.).

– The S2- incorporation in the normally-protective oxides creates lattice defects which destroy protective nature of films … tarnishing.

– Moisture not required for tarnishing, it can actually retard tarnishing of Cu in presence of traces of H2S.

• Damp atmospheres:– corrosion increases with moisture content;– at critical moisture level (~ 70% RH), an

invisible, thin film of moisture forms on (metal) surface, provides “electrolyte” for current (critical RH depends on surface condition: cleanliness, presence of oxide or scale, presence of salts or other contaminants that may be hygroscopic).

• Wet atmospheres: – promote puddles, pockets, visible water layers

(from dew, sea spray, rain, etc.)– crevices, condensation traps, etc., create water

pools, and lead to “wet atmospheric corrosion” even when rest of surface dry

– Corroded weathering steel I-beam. Note how corrosion has thinned the bottom of the vertical web where corrosion products have fallen and formed a moist corrosive deposit. soluble corrosion products increase wet corrosion (dissolved ions increase conductivity, sustain higher electrical currents)

– insoluble corrosion products may retain moisture during alternate wet and dry conditions, lead to continuous wet corrosion.

• Corroded weathering steel I-beam.

• Note how corrosion has thinned the bottom of the vertical web where corrosion products have fallen and formed a moist corrosive deposit.

Corroded steel framework on the ceiling of a parking garage. The seams in this corrugated structure act as condensation traps and lead to wet atmospheric corrosion.

• Rusting of iron and steel, formation of patina on copper, Examples of damp wet corrosion.

Corroded regions of a painted highway bridge

Corroded weathering steel highway bridge girder

ATMOSPHERIC CONTAMINANTS

• Wet atmospheric corrosion is often governed by level of contaminants.

• e.g., marine salts vary drastically with distance from the sea:

• steel at 25 m from the sea will corrode 12x faster than same steel 250 m away.

• Industrial atmospheres are generally more corrosive than rural, mainly because of sulfur compounds produced by burning fuels.

• SO2 selectively adsorbs on metals – under humid conditions metal oxide corrosion products catalyze oxidation to SO3:

SO2 + 1/2 O2 → SO3 (with a catalyst)

H2O + SO3 → H2SO4

• Small additions ( ~ 0.2%) of Cu, Ni or Cr increase resistance of steel to sulfur pollution by enhancing the formation of a tighter, more protective rust film.

NOTE: longevity of ancient Fe probably due to SO2- free

environments rather than high degree of corrosion resistance.

• Nitrogen compounds promote atmospheric corrosion - from fuel burning (NOx as well as SOx), as well as by thunderstorms.

N2 + x O2 → 2 NOx

nitrogen-based fertilizers (from NH3) increase nitrogen pollutants in atmosphere.

• H2S promotes atmospheric corrosion (e.g., Ag, Cu tarnishing)– from industry (oil & gas, pulp and paper , etc.);– from decomposition of organic S compounds;– from sulfate-reducing bacteria (SRB) in polluted rivers etc.

H2O

SRB + SO42- → H2S

• Dust particles detrimental (stick to metal surfaces, absorb water, H2SO4 etc., may contain Cl- … WHICH WHICH IS BADIS BAD … since it breaks down protective oxide films).

• CO2 dissolution in water can give pH ~ 5.6 (in equilibrium with normal atmosphere containing CO2) … BUT … CO2 is not significant in atmospheric corrosion, in fact sometimes can inhibit it (if SO2 is present).

• Surface temperature very important - as T rises, corrosion rate rises - though damp and wet corrosion stop when moisture driven off;

• Metal surfaces that retain moisture generally corrode faster than rain-washed surfaces; rain flushes impurities off surfaces, removes particles, etc. that promote differential aeration, etc.

• Winter exposure generally more severe (more combustion products in atmosphere, temperature inversions, etc.), though summer gives higher surface temperatures.

• Relative humidity very important: for clean Fe, critical RH 60% – above this, rust begins to form slowly from

deposited water film. At 75 - 80% RH, corrosion rate increases rapidly (probably because of capillary condensation within the rust layer).

– if corrosion product rust is microporous, moisture will condense at different RHs depending on pore size: • 1.5 nm - diameter pore (capillary)

condenses water at 50% RH; • 36 nm - diameter pore at 98% RH.

• Note: dust, particles, etc. on surfaces create crevices that can condense moisture at various RHs.

• Salt or soluble corrosion products will form electrolytes in condensed moisture - lower critical RH, also increase corrosion.

Damp and wet corrosion are described in terms of ELECTROCHEMISTRYELECTROCHEMISTRY.

We have seen how a metal dissolution, such as:

Zn + 2 HCl ZnCl2 + H2

can be regarded as two reactions: Zn Zn 2+ + 2 e- (oxidation - an ANODIC process)

2H+ + 2e H2 (reduction - a CATHODIC process)

BOTH REACTIONS OCCUR SIMULTANEOUSLY AND AT THE SAME RATE.

It follows, that during metallic corrosion … THE RATE OF OXIDATION EQUALS THE RATE OF

REDUCTION.

Electrochemical reactions occurring during corrosion

of zinc in air-free hydrochloric acid

• Implies that a corroding surface has anodic and cathodic areas for UNIFORM CORROSION

• These must be distributed evenly over the surface and in fact must move around.

• Some anodic “half reactions” for corrosion:

Zn Zn2+ + 2 e-

Na Na+ + e-

Fe Fe2+ + 2 e-

Cu Cu2+ + 2 e- … etc.

• Anodic “half” reaction must be balanced by cathodic “half” reactions:

• Primary cathodic “half reactions” include:

hydrogen evolution 2 H+ + 2 e- H2

oxygen reduction O2 + 4 H+ + 4 e- 2 H2O(acid solution)

oxygen reduction O2 + 2 H2O + 4 e- 4 OH-

(neutral or basic solution)

More cathodic half reactions:metal ion reduction M3+ + e- M2+

(e.g. Fe3+ + e- Fe2+)

metal deposition (e.g. Cu+ + e- Cu)

Note that the flow of charge (i.e., electrons) is a measure of the reaction rate (metal dissolution or corrosion rate).

Thus, if the corrosion “current” can be measured, the corrosion rate is directly evaluated through Faraday’s Law

FntIM

m wt

m = mass deposited/released (g);Mwt = atomic or molecular weight (g/mol);I = current passed (Amps);t = time current/potential applied (seconds);n = electrons transferred in the half-cell reaction;F = Faraday constant (96485 C/mol).

this is the number of charges that must be passed to reduce or oxidise one mole of a compound

• Say the Imeasured = 10-5 amps/cm2

• Using Faraday’s Law the corrosion rate is calculated as:

• Or in a more useable unit … (divide by the density – 7.86 g/cm3 for iron; convert cm to mm and seconds to years)

scmg109.2

molC964852cmA10molg56CRm 29

25

yr/mm12.0yr/m116CR