CORROSION, TYPES OF CORROSION AND PREVENTION

Corrosion is the deterioration of materials by chemical interaction with their environment. The term corrosion is sometimes also applied to the degradation of plastics, concrete and wood, but generally refers to metals.

Definition of Corrosion

Examples of corrosion

e.g. rusting of iron:

4 Fe + 3 O2 + 2 H2O → 4 FeOOH (lepidocrocite)

BUT: at high temperature the corrosion of iron can produce:

3 Fe + 4 H2O → Fe3O4/Fe2O3 (magnetite) + 4 H2

Sometimes, non-metals are said to corrode:e.g. degradation of graphite moderator in a CO2 cooled nuclear

reactor

C + CO2 → 2 CO (may be inhibited by addition of CH4)

Effects of corrosion

Losses are economic and safety:• Reduced Strength• Downtime of equipment• Lost surface properties• Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe, reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal.  Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small. 

Why does metal Corrode?

Energy is released as material proceeds towards “natural” state

i.e. the thermodynamically stable state.

Thus, rusting of iron is a reversion towards the (hydrated - usually) ore;

e.g., if lepidocrocite is dehydrated, we get:

2Fe OOH → Fe2O3 (haematite) + H2O

Corrosion, especially of metals, is usually an oxidative processi.e. the metal loses electrons:e.g., 4 Cu + O2 → 2 Cu2O

Zn + 2HCl → ZnCl2 + H2

Chemistry “refresher” … the Periodic Table

Valency and Ions

When atoms lose or gain electrons, they become Ions.

Cations are positive and are formed by elements on the left side of the periodic chart.

Anions are negative and are formed by elements on the right side of the periodic chart.

Valence describes the possible number of electrons an ion will pick up or donate. The valence state of a particular compound is also called its oxidation state.

In general, for metal M corroding:

M → Mz+ + z e- (oxidation)

z e- + Z/4 O2 → Z/2 O2- (reduction)

M + Z/4 O2 → MOZ/2 (combined)

For example, the corrosion of zinc in hydrochloric acid is the net result of two electrochemical reactions:

Zn → Zn2+ + 2 e-

2 HCl + 2e- → H2 + 2 Cl- (2 H+ + 2 e- → H2)

Zn + HCl → H2 + ZnCl (Zn+ + Cl-)

Question: which is the reduced species?

Classification of Corrosion

• Based on Temperature: (i) Low-temperature corrosion (ii) High-temperature corrosion• Direct combination (Oxidation) and Electrochemical

corrosion• Preferred classification: (i) Wet corrosion – Usually involves aqueous solutions or electrolytes (Steel by water) (ii) Dry corrosion – In the absence of liquid phase/moisture

Types of Corrosion

The Eight Forms of Corrosion

1. Uniform attack (general corrosion)2. Galvanic corrosion3. Crevice corrosion4. Pitting5. Intergranular 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.

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

Corroded regions of a painted highway bridge

• Remember - electrochemistry basics in aqueous solution:

• Metal dissolution is ANODIC:

M Mn+ + n e-

(e.g. Fe Fe2+ + 2 e-)

Galvanic Corrosion

• and there are several possible CATHODIC reactions:

– hydrogen evolution (acids) 2 H+ + 2 e- H2

– oxygen reduction (acids) O2 + 4H+ + 4 e- 2H2O

– oxygen reduction (neutral or base) O2 + 2 H2O + 4 e- 4 OH-

– metal ion reduction M3+ + e- M2+

– metal deposition M+ + e- M

• Note: More than one oxidation and more than one reduction reaction can occur during corrosion.

• MULTIPLE CATHODIC REACTIONS ARE IMPORTANT.

• Thus, metals tend to dissolve more readily in aerated acids than in pure, de-aerated acid:– In aerated acids, oxygen reduction AND hydrogen

evolution can occur simultaneously:2 H+ + 2 e- H2

O2 + 4H+ + 4 e- 2 H2O

– Also, an oxidizer, such as ferric ion, as an impurity in commercial acids makes them much more corrosive than pure acids because of the extra cathodic reaction that may occur:

Fe3+ + e- Fe2+

GALVANIC SERIES

• A metal in contact with a solution establishes a POTENTIAL with respect to the solution

• How would we measure the potential difference Em - Es?

• Em - Es cannot be measured, we can only measure the difference between it and Em - Es for another metal:

(Em1 - Es) - (Em2 - Es) = Em1 - Em2

This has the accepted sign convention; however, some workers use opposite sign convention. These potentials are listed in accordance with the Stockholm Convention. See J. O’M. Bockris and A. K. N. Reddy, Modern Electrochemistry, Plenum Press, New York, 2002

• EXAMPLES (from Fontana):

– A yacht with a Monel hull and steel rivets became unseaworthy because of rapid corrosion of the rivets.

– Severe attack occurred on aluminum tubing connected to brass return bends.

– Domestic hot-water tanks made of steel fail where copper tubing is connected to the tank.

– Pump shafts and valve stems made of steel or more corrosion-resistant materials fail because of contact with graphite packing.

Narrow openings, gaps, spaces, pores etc. between metal-metal components or metal-non-metal components may provoke localized corrosion

NOTE: unintentional crevices (seams, cracks etc.) can also act in the same way

Passive alloys (especially stainless steels) are more vulnerable than more active alloys.

Crevice Corrosion

Crevice corrosion at a metal-to-metal crevice site formed between components of type 304 stainless steel fastener in seawater

Pitting is an insidious and destructive form of corrosion:

– difficult to detect (pits may be small on surface, but extensive below surface from undercutting; may be covered with deposit);

– can cause equipment to fail (by perforation) with very little weight loss;

– difficult to measure as pit depth and distribution vary widely under (nominally) identical conditions;

– “incubation” period may be months or years.

Pitting Corrosion

Examples of pitting corrosion:

– Alloy-800 SG tubes with phosphate chemistry…pitting severe pitting wastage

– SS cooling water left static under silted conditions…severe pitting; replaced with Ti plate-type

– Others?

SELECTIVE LEACHING (“Dealloying”, “Parting”)

Corrosion in which one constituent of an alloy is preferentially removed, leaving behind an altered (weakened) residual structure.

Can occur in several systems.

Dezincification (Characteristics)

All Cu-Zn alloys (Brasses) containing > 15% Zn are susceptible . . .

e.g. common yellow brass . . . 30 % Zn &70 % Cu, dezincifies to red copper-rich structure. Dezincification can be uniform...

- potable water inside

- or plug-type.... (boiler water inside, combustion gases outside)

Uniform dezincification of brass pipe.

Plug-type dezincification.

EROSION-CORROSION (“Flow-Assisted” or “Flow-Accelerated” Corrosion)

An increase in corrosion brought about by a high relative velocity between the corrosive environment and the surface.

Removal of the metal may be:– as corrosion product which “spalls off” the surface because of the

high fluid shear and bares the metal beneath;– as metal ions, which are swept away by the fluid flow before they

can deposit as corrosion product.

The distinction between erosion-corrosion and erosion:– erosion is the straightforward wearing away by the mechanical

abrasion caused by suspended particles . . . e.g., sand-blasting, erosion of turbine blades by droplets . . .

erosion-corrosion also involves a corrosive environment . . . the metal undergoes a chemical reaction.

Erosion-corrosion produces a distinctive surface finish:

grooves, waves, gullies, holes, etc., all oriented with respect to the fluid flow pattern . . . “scalloping”...

Erosion-corrosion of condenser tube wall.

Erosion-corrosion of stainless alloy pump impeller.

Impeller lasted ~ 2 years in oxidizing conditions;after switch to reducing conditions, it lasted ~ 3 weeks!

STRESS CORROSION (“Stress Corrosion Cracking” - SCC)

Under tensile stress, and in a suitable environment,

some metals and alloys crack . . . usually, SCC noted

by absence of significant surface attack . . . occurs in

“ductile” materials.

“Transgranular” SCC (“TGSCC”)

Cross section of stress-corrosioncrack in stainless steel.

“Intergranular” SCC (“IGSCC”)

Intergranular stress corrosion cracking of brass.

Two original classic examples of SCC:– “season cracking” of brass;– “caustic embrittlement” of CS;

“Season Cracking”

Occurs where brass case is crimped onto bullet, i.e., in area of high residual stress.

Common in warm, wet environments (e.g., tropics).

Ammonia (from decomposition of organic matter, etc.) must be present.

Season cracking of German ammunition.

“Caustic Embrittlement”Early steam boilers (19th and early 20th century) of riveted carbon steel.

Both stationary and locomotive engines often exploded.

Examination showed:• cracks or brittle failures around rivet holes;• areas susceptible were cold worked by riveting (i.e., had high residual

stresses);• whitish deposits in cracked regions were mostly caustic (i.e., sodium

hydroxide from chemical treatment of boiler water); • small leaks at rivets would concentrate NaOH and even dry out to solid.

SCC revealed by dye penetrant.

Carbon steel plate from a caustic storage tank failed by caustic embrittlement.

Corrosion Fatigue The fatigue fracture of a metal aggravated by a corrosive environment or the stress corrosion cracking of a metal aggravated by cyclic stress.

N.B. Fatigue fracture usually occurs at stresses below the yield point but after many cyclic applications of the stress.

Typical “S-N” curves:

Fatigue-fractured material often shows most of the fracture face shiny metallic, with the final area to fracture (mechanically by brittle fracture of a reduced cross-section) having a rough crystalline appearance . . .

If corrosion-fatigue occurs, the “shiny-metallic” area might be covered with corrosion products; BUT normal fatigue fractures may also develop corrosion products - depends on environment, stress pattern, etc.

N.B. In normal fatigue, the frequency of the stress cycles is not important. (can do accelerated fatigue tests at high frequency - the total number of cycles determines fatigue).

BUT in corrosion fatigue, low-cycle stresses are more damaging than high-frequency stresses. (Assignment)

Environment is important….e.g., in seawater:• Al bronzes and type 300 series SS lose 20-30% of normal fatigue resistance;• high-Cr alloys lose 60-70% resistance.

N.B. Cyclic loads mean lower allowable stresses, this must be designed into components; if there is also a corrosive environment, the allowable stresses are EVEN LOWER.

Prevention of Corrosion Fatigue

• change design so as to reduce stress and/or cycling.• reduce stress by heat treatment (for residual stress), shot

peening (to change surface residual stresses to COMPRESSIVE).

• use corrosion inhibitor with care!• use coatings . . . electrodeposited

– Zn;– Cr;– Ni;– Cu;

and– nitrided layers (heating of steels in contact with N-containing

material e.g., NH3, NaCN, etc.).