Stress - Corrosion Cracking and Hydrogen Cracking

19.2.5 Stress - Corrosion Cracking and Hydrogen Cracking

In the presence of an applied or residual tensile stress, stainless steels may crack transgranularly when exposed to certain environments (Fig. 19.7 ). Compressive stresses, to the contrary, are not damaging. The higher the tensile stress, the shorter is the time to failure. Although at low stress levels, time to cracking may

be long, there is, in general, no practical minimum stress below which cracking will not occur, given suffi cient time in a critical environment. Damaging environments that cause cracking may differ for austenitic, com- pared to martensitic or ferritic, stainless steels. For austenitic steels, the two major damaging ions are hydroxyl and chloride (OH − and Cl − ). A boiling, rela- tively concentrated chloride that hydrolyzes to a slightly acid pH, such as FeCl 2 or MgCl 2 , can cause cracking of thick sections of stressed 18 – 8 within hours. A

* In aerated 4% NaCl, maximum weight loss by pitting occurs at 90 ° C. [H. Uhlig and M. Morrill, Ind. Eng. Chem.

33 , 875 (1941)]. To avoid S.C.C. and to minimize pitting, operating temperatures in NaCl solutions should be kept below 60 – 80 ° C.

STAINLESS STEELS

Figure 19.7. Stress-corrosion cracking of 18 –8, type 304 stainless steel exposed to calcium silicate insulation containing 0.02 –0.5% chlorides, 100 °C (250 ×). Note that the cracks for this environment start at a pit. An undulating path accounts for disconnected cracks as viewed in one plane [47]. ( Copyright ASTM International. Reprinted with permission .)

solution of concentrated MgCl 2 boiling at about 154 ° C, for example, is used as an accelerated test medium. The presence of dissolved oxygen in such solutions is not necessary for cracking to occur, but its availability hastens damage, as does the presence of oxidizing ions, such as Fe 3+ . Pitting is not a preliminary require-

ment for initiation of cracks. In NaCl and similar neutral solutions, on the other hand, cracking of austenitic stainless steels is observed only if dissolved oxygen or other oxidizing agent is present [46] , and the amount of chloride needed to cause damage can be extremely small (Fig. 19.8 ). Cracking of 18 – 8 stainless tubes in heat exchangers has been observed in practice after contact with cooling waters containing 25 ppm Cl − or less, and cracking has also been induced by small

amounts of chlorides contained in magnesia insulation wrapped around stainless - steel tubes [47] . In these instances, if small pits form initially at which chlorides concentrate (Figs. 19.5 and 19.7 ) (see Section 19.2.4.1 ), the cracking tendency is accentuated by concentrated FeCl 2 and analogous metal chlorides within the pits. Hence, oxygen may induce stress - corrosion cracking in sodium chloride solutions because pitting occurs when it is present. Another contributing factor is a shift of the corrosion potential in the presence of oxygen, but not in its absence, to values that are noble to the critical potential for stress - corrosion cracking. For this situation, stress - corrosion cracking can occur regardless of whether corrosion pits develop.

Cracking by alkaline solutions requires relatively high concentrations of OH − ; hence, cracking of 18 – 8 usually occurs not in alkaline boiler water, but

356 ALLOYING FOR CORROSION RESISTANCE; STAINLESS STEELS

Figure 19.8. Relation between chloride and oxygen content of boiler water on stress - corrosion cracking of austenitic 18 – 8 - type stainless steels exposed to steam phase with inter- mittent wetting; pH 10.6, 50 ppm PO 3 4 − , 242 – 260 ° C (467 – 500 ° F), 1 – 30 days ’ exposure [W. Williams and J. Eckel, J. Am. Soc. Nav. Eng. 68 , 93 (1956)].

rather within the splash zone above the water line where dissolved alkalies con- centrate by evaporation. Failures can occur in such solutions in the absence of dissolved oxygen [48] . There is no evidence that transgranular stress - corrosion cracking occurs in pure water or pure steam.

Cracking of stressed, nickel - free ferritic steels, in general, does not occur in the chloride media described previously, these steels being suffi ciently resistant to warrant their practical use in preference to austenitic stainless steels in chlo-

ride - containing solutions. They (e.g., type 430) are also resistant to 55% Ca(NO 3 2 ) solution boiling at 117 ° C and to 25% NaOH solution boiling at 111 ° C [49] . When alloyed with more than 1.5% Ni, the 18% Cr – Fe, 0.003% C stainless steels, cold -

rolled, are susceptible to transgranular stress - corrosion cracking in MgCl 2 boiling at 130 ° C. When annealed at 815 ° C for 1 h, only the alloy containing 2% Ni fails; both higher and lower nickel contents resist cracking up to 200 h [50] . The effect of nickel is partly explained by an observed shift of potentials such that, at and above 1.5% Ni, the corrosion potential of the cold - rolled material becomes noble

STAINLESS STEELS

to the critical potential; hence cracking occurs. The reverse order of potentials applies to alloys of lower nickel content [50] .

In slightly or moderately acidic solutions, the martensitic steels, when heat - treated to high hardness values, are very sensitive to cracking, particularly in the presence of sulfi des, arsenic compounds, or oxidation products of phosphorus or selenium. The specifi c anions of the acid make little difference so long as hydro- gen is evolved, contrary to the situation for austenitic steels, for which only spe- cifi c anions are damaging. Also, cathodic polarization, rather than protecting against cracking, accelerates failure. All these facts suggest that the martensitic steels under these conditions fail not by stress - corrosion cracking but by hydro- gen cracking (see Section 8.4 ). The more ductile ferritic stainless steels undergo hydrogen blistering instead when they are cathodically polarized in seawater, especially at high current densities. Austenitic stainless steels are immune to both hydrogen blistering and cracking.

Galvanic coupling of active metals to martensitic stainless steels may also lead to failure because of hydrogen liberated on the stainless (cathodic) surface. Such failures have been demonstrated in laboratory tests [51] . As mentioned in

Section 8.4 , practical instances have been noted of self - tapping martensitic stainless - steel screws cracking spontaneously soon after being attached to an aluminum roof in a seacoast atmosphere. Similarly, hardened martensitic stain- less - steel propellers coupled to the steel hull of a ship have failed by cracking soon after being placed in service. Severely cold - worked 18 – 8 austenitic stainless steels may also fail under conditions that would damage the martensitic types

[52] . Here again, sulfi des accelerate damage, and since the alloy on cold working undergoes a phase transformation to ferrite, the observed effect is probably another example of hydrogen cracking.

Martensitic stainless steels and also the precipitation - hardening types, heat - treated to approximately > 200,000 psi ( > 1,400 MPa) yield strength, have been reported to crack spontaneously in the atmosphere, in salt spray, and immersed in aqueous media, even when not coupled to other metals [53 – 55] . Martensitic stainless - steel blades of an air compressor [56] have failed along the leading edges where residual stresses were high and where condensation of moisture occurred. Stressed to about 75% of the yield strength, and exposed to a marine atmosphere, life of ultra - high - strength 12% Cr martensitic stainless steels was in the order of

10 days or less [57] . It is generally accepted that ultra - high strength steels crack by a hydrogen embrittlement mechanism [55] .

Austenitic stainless steels containing more than about 45% Ni are immune to stress - corrosion cracking in boiling MgCl 2 solution and probably in other chloride solutions as well (Fig. 19.9 ) [58] . Ede- leanu and Snowden [48] noted that high - nickel stainless steels were more resis- tant to cracking in alkalies. Increasing the amount of nickel in austenitic stainless

19.2.5.1 Metallurgical Factors.

steels shifts the critical potential for stress - corrosion cracking in MgCl 2 solution in the noble direction more rapidly than it shifts the corresponding corrosion

358 ALLOYING FOR CORROSION RESISTANCE; STAINLESS STEELS

Figure 19.9. Stress - corrosion cracking of 15 – 26% Cr – Fe – Ni alloy wires in boiling 42% MgCl 2 . Low - nickel or nickel - free alloys are ferritic and do not crack. Austenitic alloys do not crack above 45% Ni [58] .

potential; hence, the alloys become more resistant [59] . At and above approxi- mately 45% Ni, the alloys resist stress - corrosion cracking regardless of applied potential, indicating that not environmental, but metallurgical factors, such as unfavorable dislocation arrays or decreasing interstitial nitrogen solubility, become more important.

For austenitic steels that are resistant to transformation on cold working (e.g., type 310), nitrogen is the element largely responsible for stress - cracking susceptibility, whereas additions of carbon decrease susceptibility (Fig. 19.10 )

[60] . The effect is related to alloy imperfection structure rather than to any shift of either critical or corrosion potential [59] . Stabilizing additions effective in preventing intergranular corrosion, such as titanium or columbium, have no

STAINLESS STEELS

Figure 19.10. Stress - corrosion cracking of cold - rolled 19% Cr, 20% Ni austenitic stainless steels in boiling MgCl 2 (154 ° C) as affected by carbon or nitrogen content [60] .

benefi cial effect on stress - corrosion cracking, nor do alloying additions such as

2 – 3% Mo in type 316 stainless steel. Chloride S.C.C. occurs primarily above about 90 ° C (190 ° F), but, in acidifi ed chloride solutions, S.C.C. can occur at lower temperatures [61, 62] .