Chapter 17-1

CORROSION AND DEGRADATION

Chapter 17-2

• Corrosion: --the destructive electrochemical attack of a material. --Al Capone's ship, Sapona, off the coast of Bimini.

• Cost: --4 to 5% of the Gross National Product (GNP)* --this amounts to just over $400 billion/yr**

Photos courtesy L.M. Maestas, Sandia National Labs. Used with permission.

THE COST OF CORROSION

Chapter 17-2

4Fe + 6H2O + 3O2 4Fe(OH)3

 gives ferric hydroxide 2Fe(OH)3 Fe2O3 3H2O

gives iron oxide (rust) and water

Basic “rusting” or corrosion requirements1. The metal is oxidized at the anode of an electrolytic

cell2. Some ions are reduced at the cathode3. There is a potential or voltage difference between the

anode and cathode4. An electrolyte (fluid) must be present5. The electrical path must be completed

The Rusting Mechanism (Peel)

Chapter 17-2

• Material properties• Metallurgical factors• Passivity• Environment

Metallurgical factors• Chemical segregation• Presence of multiple

phases• Inclusions• Cold Work• Non-uniform stresses

Factors affecting Corrosion (Peel)

Passivity• Example with steel in

nitric acid…dilute solutions will cause rapid attack, strong solutions have little visible effect.

• Surface film can be formed

• Some types of steel may do this with rust

• Aluminum does this• Need to watch passive

film, but can be used for simple protection

Chapter 17-

• Ranks the reactivity of metals/alloys in seawaterm

ore

anodic

(a

ctiv

e)

more

cath

odic

(i

nert

)

PlatinumGoldGraphiteTitaniumSilver316 Stainless SteelNickel (passive)CopperNickel (active)TinLead316 Stainless SteelIron/SteelAluminum AlloysCadmiumZincMagnesium

8

Based on Table 17.2, Callister 6e. (Source of Table 17.2 is M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Company, 1986.)

GALVANIC SERIES

Chapter 17-9

Forms of

corrosion

• Uniform AttackOxidation & reductionoccur uniformly oversurface.

• Selective LeachingPreferred corrosion ofone element/constituent(e.g., Zn from brass (Cu-Zn)).

• IntergranularCorrosion alonggrain boundaries,often where specialphases exist.

• Stress corrosionStress & corrosionwork togetherat crack tips.

• GalvanicDissimilar metals arephysically joined. Themore anodic onecorrodes.(see Table17.2) Zn & Mgvery anodic.

• Erosion-corrosionBreak down of passivatinglayer by erosion (pipeelbows).

• PittingDownward propagationof small pits & holes.

• Crevice Between twopieces of the same metal.

Rivet holes

attacked zones

g.b. prec.

Fig. 17.6, Callister 6e. (Fig. 17.6 is courtesy LaQue Center for Corrosion Technology, Inc.)

Fig. 17.9, Callister 6e.

Fig. 17.8, Callister 6e.(Fig. 17.8 from M.G.Fontana, CorrosionEngineering, 3rd ed.,McGraw-Hill BookCompany, 1986.)

TYPES OF CORROSION

Chapter 17-

Uniform attack• Uniform Attack Uniform attack is the most common form of corrosion. It is normally

characterized by a chemical or electrochemical reaction which proceeds uniformly over the entire exposed surface or over a large area. The metal becomes thinner and eventually fails. For example, a piece of steel or zinc immersed in dilute sulfuric acid will normally dissolve at a uniform rate over its entire surface. A sheet iron roof will show essentially the same degree of rusting over its entire outside surface. Uniform attack, or general overall corrosion, represents the greatest destruction of metal on a tonnage basis. This form of corrosion, however, is not of too great concern from the technical standpoint, because the life of equipment can be accurately estimated on the basis of comparatively simple tests. Merely immersing specimens in the fluid involved is often sufficient. Uniform attack can be prevented or reduced by (1) proper materials, including coatings, (2) inhibitors, or (3) cathodic protection.

Chapter 17-

Uniform Attack

•

Chapter 17-

Stress Corrosion

• Stress-corrosion cracking refers to cracking caused by the simultaneous presence of tensile stress and a specific corrosive medium. Many investigators have classified all cracking failures occurring in corrosive mediums as stress-corrosion cracking, including failures due to hydrogen embrittlement. However, these two types of cracking failures respond differently to environmental variables. To illustrate, cathodic protection is an effective method for preventing stress-corrosion cracking whereas it rapidly accelerates hydrogen-embrittlement effects. Hence, the importance of considering stress-corrosion cracking and hydrogen embrittlement as separate phenomena is obvious. During stress-corrosion cracking, the metal or alloy is virtually unattacked over most of its surface, while fine cracks progress through it. This cracking phenomenon has serious consequences since it can occur at stresses within the range of typical design stress. Exposure to boiling MgCl2 at 310°F (154°C) is shown to reduce the strength capability to approximately that available at 1200°F.

Chapter 17-

Stress Corrosion• The two classic cases of stress-corrosion cracking are "season cracking"

of brass, and the "caustic embrittlement" of steel. Both of these obsolete terms describe the environmental conditions present which led to stress-corrosion cracking. Season cracking refers to the stress-corrosion cracking failure of brass cartridge cases. During periods of heavy rainfall, especially in the tropics, cracks were observed in the brass cartridge cases at the point where the case was crimped to the bullet. It was later found that the important environmental component in season cracking was ammonia resulting from the decomposition of organic matter.

• Many explosions of riveted boilers occurred in early steam-driven locomotives. Examination of these failures showed cracks or brittle failures at the rivet holes. These areas were cold-worked during riveting operations, and analysis of the whitish deposits found in these areas showed caustic, or sodium hydroxide, to be the major component. Hence, brittle fracture in the presence of caustic resulted in the term caustic embrittlement. While stress alone will react in ways well known in mechanical metallurgy (i.e., creep, fatigue, tensile failure) and corrosion alone will react to produce characteristic dissolution reactions; the simultaneous action of both sometimes produces the disastrous results.

Chapter 17-

Stress Corrosion

Chapter 17-

Erosion Corrosion

• Erosion corrosion is the acceleration or increase in rate of deterioration or attack on a metal because of relative movement between a corrosive fluid and the metal surface. Generally, this movement is quite rapid, and mechanical wear effects or abrasion are involved. Metal is removed from the surface as dissolved ions, or it forms solid corrosion products which are mechanically swept from the metal surface. Sometimes, movement of the environment decreases corrosion, particularly when localized attack occurs under stagnant conditions, but this is not erosion corrosion because deterioration is not increased.

• Erosion corrosion is characterized in appearance by grooves, gullies, waves, rounded holes, and valleys and usually exhibits a directional pattern. In many cases, failures because of erosion corrosion occur in a relatively short time, and they are unexpected largely because evaluation corrosion tests were run under static conditions or because the erosion effects were not considered

Chapter 17-

Erosion Corrosion

Chapter 17-

Selective Leaching

• Selective leaching is the removal of one element from a solid alloy by corrosion processes. The most common example is the selective removal of zinc in brass alloys (dezincification). Similar processes occur in other alloy systems in which aluminum; iron, cobalt, chromium, and other elements are removed. Selective leaching is the general term to describe these processes, and its use precludes the creation of terms such as dealuminumification, decobaltification, etc. Parting is a metallurgical term that is sometimes applied, but selective leaching is preferred.

Chapter 17-

Corrosion Fatigue

• It may be described as the reduction of fatigue strength by the corrosive environment.

• This type of corrosion occurs by the simultaneous effect of environment and cyclic fluctuations of stress, leading to early failure of the component.

• The effect of corrosion on S-N Curve

Chapter 17-

The effect of corrosion on S-N Curve

Chapter 17-

Intergranular Corrosion

• Grain boundary effects are of little or no consequence in most applications or uses of metals. If a metal corrodes, uniform attack results since grain boundaries are usually only slightly more reactive than the matrix. However, under certain conditions, grain interfaces are very reactive and intergranular corrosion results. Localized attack at and adjacent to grain boundaries, with relatively little corrosion of the grains, is intergranular corrosion. The alloy disintegrates (grains fall out) and/or loses its strength.

• Intergranular corrosion can be caused by impurities at the grain boundaries, enrichment of one of the alloying elements, or depletion of one of these elements in the grain-boundary areas. Small amounts of iron in aluminum, wherein the solubility of iron is low, have been shown to segregate in the grain boundaries and cause intergranular corrosion. It has been shown that based on surface tension considerations the zinc content of a brass is higher at the grain boundaries. Depletion of chromium in the grain-boundary regions results in intergranular corrosion of stainless steels.

Chapter 17-

Pitting

• Pitting is a form of extremely localized attack that results in holes in the metal. These holes may be small or large in diameter, but in most cases they are relatively small. Pits are sometimes isolated or so close together that they look like a rough surface. Generally a pit may be described as a cavity or hole with the surface diameter about the same as or less than the depth.

Pitting is one of the most destructive and insidious forms of corrosion. It causes equipment to fail because of perforation with only a small percent weight loss of the entire structure. It is often difficult to detect pits because of their small size and because the pits are often covered with corrosion products. In addition, it is difficult to measure quantitatively and compare the extent of pitting because of the varying depths and numbers of pits that may occur under identical conditions. Pitting is also difficult to predict by laboratory tests. Sometimes the pits require a long time-several months or a year-to show up in actual service. Pitting is particularly vicious because it is a localized and intense form of corrosion, and failures often occur with extreme suddenness

Chapter 17-10

• Self-protecting metals! --Metal ions combine with O2

to form a thin, adhering oxide layer that slows corrosion.

Metal (e.g., Al, stainless steel)

Metal oxide

• Reduce T (slows kinetics of oxidation and reduction)• Add inhibitors --Slow oxidation/reduction reactions by removing reactants (e.g., remove O2 gas by reacting it w/an inhibitor). --Slow oxidation reaction by attaching species to the surface (e.g., paint it!).• Cathodic (or sacrificial) protection --Attach a more anodic material to the one to be protected.

Adapted from Figs. 17.13(a), 17.14 Callister 6e. (Fig. 17.13(a) is from M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Co., 1986.)

CONTROLLING CORROSION

steel

zinczincZn2+

2e- 2e-

e.g., zinc-coated nail

steel pipe

Mg anode

Cu wiree-

Earth

Mg2+

e.g., Mg Anode

Chapter 17-11

• Corrosion occurs due to: --the natural tendency of metals to give up electrons. --electrons are given up by an oxidation reaction. --these electrons then are part of a reduction reaction.• Metals with a more negative Standard Electrode Potential are more likely to corrode relative to other metals.• The Galvanic Series ranks the reactivity of metals in seawater.• Increasing T speeds up oxidation/reduction reactions.• Corrosion may be controlled by: -- using metals which form a protective oxide layer -- reducing T

-- adding inhibitors-- painting--using cathodic protection.

SUMMARY