CORROSION FATIGUE

8.6 CORROSION FATIGUE

A metal that progressively cracks on being stressed alternately (reverse bending) or repeatedly is said to fail by fatigue. The greater the applied stress at each cycle, the shorter is the time to failure. A plot of stress versus number of cycles to failure, called the S – N curve, is shown in Fig. 8.15 . A number of cycles at the correspond- ing stress to the right of the upper solid line results in failure, but no failure occurs for an infi nite number of cycles at or below the endurance limit or fatigue limit . For steels, but not necessarily for other metals, a true endurance limit exists which is approximately half the tensile strength. The fatigue strength of any metal, on the other hand, is the stress below which failure does not occur within a stated number of cycles. Frequency of stress application is sometimes also stated because this factor may infl uence the number of cycles to failure.

In general, a corrosive environment can decrease the fatigue properties of any engineering alloy, meaning that corrosion fatigue is not dependent on mate- rial and environment [73] . In a corrosive environment, failure at a given stress level usually occurs within fewer cycles, and a true fatigue limit is not observed (Fig. 8.15 ). In other words, failure occurs at any applied stress if the number of cycles is suffi ciently large. Cracking of a metal resulting from the combined action of a corrosive environment and repeated or alternate stress is called corrosion

174 EFFEC T OF STRESS

Figure 8.15. S – N curve for steels subjected to cyclic stress.

Figure 8.16. Corrosion - fatigue crack through mild steel sheet, resulting from fl uttering of the sheet in a fl ue gas condensate (250 × ).

fatigue . The damage is almost always greater than the sum of the damage by cor- rosion and fatigue acting separately.

Corrosion - fatigue cracks are typically transgranular. They are often branched (Fig. 8.16 ), and several cracks are usually observed at the metal surface in the vicinity of the major crack accounting for failure. Fatigue cracks are similarly transgranular (exception: lead and tin), but rarely is there evidence of more than

CORROSION FATIGUE

one major crack. In corrosion fatigue, corrosion pits may form at the metal surface at the base of which cracks initiate, but pitting is not a necessary precur- sor to failure.

Aqueous environments causing corrosion fatigue are numerous and are not specifi c, contrary to the situation for S.C.C., for which only certain ion – metal combinations result in damage. Corrosion fatigue of steel occurs in fresh waters, seawater, combustion - product condensates, general chemical environments, and so on, with the general rule that the higher the uniform corrosion rate, the shorter the resultant fatigue life.

Corrosion fatigue is a common cause of unexpected cracking of vibrating metal structures designed to operate safely in air at stresses below the fatigue limit. For example, the shaft of a ship propeller slightly out of line will operate satisfactorily until a leak develops, allowing water to impinge on the shaft in the area of maximum alternating stress. Cracks may then develop within a matter of days, resulting in eventual parting and failure of the shaft. Steel oil - well sucker rods, used to pump oil from underground, have limited life because of corrosion fatigue resulting from exposure to oil - well brines. Despite use of high - strength medium - alloy steels and oversized rods, failures from this source are a loss to the oil industry in the order of millions of dollars annually. Wire cables commonly fail by corrosion fatigue. Pipes carrying steam or hot liquids of variable tempera- ture may fail similarly because of periodic expansion and contraction (thermal cycling).

The usual fatigue test conducted in air on a structural metal is infl uenced by both oxygen and moisture and, in part, therefore, always represents a measure of corrosion fatigue. In early tests, the fatigue strength for copper in a partial vacuum was found to be increased 14% over that in air. For mild steel the increase was only 5%, but for 70 – 30 brass it was 26% [74] . In later tests [75] , fatigue life of oxygen - free, high - conductivity (OFHC) copper at 10 −5

mmHg air pressure was found to be 20 times greater than at 1 - atm air pressure. The main effect was attributed to oxygen; this had little effect on initiation of cracks, but had considerable effect on rate of crack propagation. Fatigue life of pure alumi- num was also affected by air; but contrary to the situation for copper, water vapor in absence of air was equally effective. Gold, which neither chemisorbs oxygen nor oxidizes, had the same life whether fatigued in air or in a vacuum.

In some environments, fatigue cracking is supplemented by S.C.C., with the latter occurring under conditions of constant stress. This is seen in the behavior of a high - strengh low - alloy steel fatigued in both the absence and presence of

moisture. Steels of greater than about 1140 MPa (165 ksi) yield strength (R c 37) are subject to S.C.C. in water at room temperature and have shorter fatigue life in moist air compared to dry air; whereas steels of lower strength, which do not undergo S.C.C. in water, have the same fatigue life (Fig. 8.17 ) [76] .

Fresh waters and particularly brackish waters have a greater effect on the corrosion fatigue of steels than on that of copper, with the latter being a more corrosion - resistant metal. Stainless steels and nickel or nickel - base alloys are also better than carbon steels. In general, resistance of a metal to corrosion fatigue is

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Figure 8.17. Tensile strength, yield strength, fatigue limit in dry air, and fatigue strength at 10 7 cycles in 93% relative humidity air, of 4140 steel heat - treated to various hardness values [76] . (With kind permission of Springer Science and Business Media.)

associated more nearly with its inherent corrosion resistance than with high mechanical strength .

A few values of corrosion fatigue strength determined by McAdam [77] in fresh and brackish waters are listed in Table 8.5 . These values, besides varying with environment, are found to vary with rate of stressing, with temperature, and with aeration; hence, they are useful only for qualitative comparison of one metal with another. Unlike the fatigue limit in air, they are not usually reli- able for engineering design. Conclusions from data of Table 8.5 and similar data are as follows:

1. There is no relation between corrosion fatigue strength and tensile strength.

2. Medium - alloy steels have only slightly higher corrosion fatigue strength than carbon steels.

CORROSION FATIGUE

T A B L E 8.5. Fatigue Limit and Corrosion Fatigue Strength of Various Metals [77] Metal

Fatigue

Corrosion Damage Ratio

Limit

Fatigue Strength (Corrosion

in Air

(psi a ) Fatigue

(psi a )

Strength)/ (Fatigue Limit)

Well

Salt Well Salt

Water

b Water c Water Water (10 7 – 10 8 cycles at 1450 cycles/min)

0.64 0.16% C steel, quenched and tempered

0.11% C steel, annealed

7,000 0.57 0.2 1.09% C steel, annealed

0.55 3.5% Ni, 0.3% C steel, annealed

0.59 0.9% Cr, 0.1% V, 0.5% C steel, annealed

0.52 13.8% Cr, 0.1% C steel, quenched and

18,000 0.70 0.36 tempered 17% Cr, 8% Ni, 0.2% C steel, hot - rolled

25,000 1.00 0.50 Nickel, 98.96%, annealed 760 ° C

21,500 0.71 0.65 Monel, 67.5% Ni, 29.5% Cu, annealed

28,000 0.71 0.77 760 ° C Cupro - nickel, 21% Ni, 78% Cu, annealed

18,000 0.95 0.95 760 ° C Copper, annealed 650 ° C

10,000 1.02 1.02 Aluminum, 99.4%, annealed

2,100 0.36 Aluminum, 98%, 1.2% Mn, hard

3,800 0.51 0.36 Duralumin, tempered

6,500 0.45 0.38 Brass, 60 – 40, annealed

0.86 a To convert from psi to MPa, multiply by 6890.

b 2 ppm CaSO 4 , 200 ppm CaCO 3 , 17 ppm MgCl 2 , 140 ppm NaCl.

c Severn River water, with about one - sixth the salinity of seawater.

3. Heat treatment does not improve corrosion fatigue strength of either carbon or medium - alloy steels; residual stresses are deleterious.

4. Corrosion - resistant steels, particularly steels containing chromium, have higher corrosion fatigue strength than other steels.

5. Corrosion fatigue strength of all steels is lower in salt water than in fresh water.

8.6.1 Critical Minimum Corrosion Rates

In order for the corrosion process to affect fatigue life, the corrosion rate must exceed a minimum value. Such rates are conveniently determined by anodic polarization of test specimens in deaerated 3% NaCl, translating uniform

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T A B L E 8.6. Critical Minimum Corrosion Rates, 25 ° C,

30 Cycles/s

Reference

μ A/cm 2 a gmd

0.18% C mild steel 2.0 0.50 78 4140 low alloy steel c 20 R

2.5 0.63 76 c 37 R

2.0 0.50 76 c 44 R

2.8 0.70 76 Nickel

1.2 < 0.30 79 OFHC copper

a To convert μ A/cm 2 to A/m 2 , divide by 100.

current densities by Faraday ’ s Law into corrosion rates below which the fatigue life is unaffected. These measured current densities are found to be indepen- dent of the total area of specimen surface exposed to anodic currents. Various values at 30 cycles/s (1800 cycles/min) are listed in Table 8.6 . It is expected that such values increase with cyclic frequency of the test. For the steels, critical corrosion rates are independent of (a) carbon content, (b) applied stress below the fatigue limit, and (c) heat treatment (hardness). The average value of

0.58 gmd (5.8 mdd) is less than the uniform corrosion rates of steels in aerated water and 3% NaCl (1 – 10 gmd, 10 – 100 mdd). But at pH 12, the uniform corro- sion rate falls below the critical rate, and the fatigue life regains its value in air

[78] . The existence of a critical rate in 3% NaCl explains why cathodic protec- tion of steel against corrosion fatigue requires polarizing to only − 0.49 V (S.H.E.), whereas the value required to attain zero corrosion rate lies 40 mV more active at − 0.53 V.

For copper, the critical rate of 28.5 gmd (285 mdd) is much higher than the uniform corrosion rates in aerated water and 3% NaCl (0.4 – 1.5 gmd, 4 – 15 mdd); hence, the fatigue life of copper is observed to be about the same in air as in fresh and saline waters (Table 8.5 ).

8.6.2 Remedial Measures

There are several means available for reducing corrosion fatigue. In the case of mild steel, thorough deaeration of a saline solution restores the normal fatigue limit in air (Fig. 8.18 ) [81] . Cathodic protection to − 0.49 V (S.H.E.) accomplishes the same result. Inhibitors are also effective [82, 83] . Sacrifi cial coatings (e.g., zinc or cadmium electrodeposited on steel) are effective because they cathodically protect the base metal at defects in the coating. One of the very fi rst observations and diagnoses of corrosion fatigue, made by B. Haigh in about 1916, involved premature failure of steel towing cables exposed to seawater, and galvanizing

CORROSION FATIGUE

Figure 8.18. Effect of dissolved oxygen concentration in 3% NaCl, 25 ° C, on fatigue behavior of 0.18% C steel [81] . (Reprinted with permission of ASM International ® . All rights reserved. www.asminternational.org .)

provided greatly increase life in this application [84] . Electrodeposits of tin, lead, copper, or silver on steel are also said to be effective without reducing normal fatigue properties; presumably, they owe their effectiveness to exclusion of the environment [85] . Organic coatings are useful if they contain inhibiting pigments in the prime coat. Shot peening the metal surface, or otherwise introducing com- pressive stresses, is benefi cial.

8.6.3 Mechanism of Corrosion Fatigue

The mechanism of fatigue in air proceeds by localized slip within grains of the metal caused by alternating stress, resulting in slip steps at the metal surface. Adsorption of air on the clean metal surface exposed at slip steps probably pre- vents rewelding on the reverse stress cycle. Continued slip produces displaced clusters of slip bands, which protrude above the metal surface (extrusions); cor- responding incipient cracks (intrusions) form elsewhere (Fig. 8.19 ). Below the fatigue limit, work hardening accompanying each cycle of plastic deformation eventually impedes further slip, which in turn impedes the fatigue process.

The basic effect of the corrosion process is to accelerate plastic deformation accompanied by formation of extrusions and intrusions. For this reason, damage by corrosion fatigue — a conjoint action of corrosion and fatigue — is greater than the damage caused by the sum of both acting separately. In addition, corrosion resistance of a metal is usually more important than high tensile strength in

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Figure 8.19. Extrusions and intrusions in copper after 6 × 10 5 cycles in air. Specimen plated with silver after test and mounted at an angle to magnify surface protuberances by about 20 × . Overall magnifi cation about 500 × . [Figure 3 from W. Wood and H. Bendler, The fatigue process in copper as studied by electron metallography, Trans. Metall. Soc. AIME 224 , 182 (1962).]

establishing optimum resistance to corrosion fatigue. Since pure metals are not immune to uniform corrosion, they are also not immune to corrosion fatigue.

The mechanism of the slip dissolution process [86] takes place in the follow- ing steps:

Diffusion of the active species to the crack tip

Rupture of the protective fi lm at the slip step

Dissolution of the exposed surface

Nucleation and growth of the protective fi lm on the bare surface