Cracking of Sensitized Austenitic Alloys in Polythionic Acids

19.2.6 Cracking of Sensitized Austenitic Alloys in Polythionic Acids

During shutdown of certain 18 – 8 stainless - steel oil - refi nery equipment operating in the carbide precipitation temperature range, it was found that rapid inter- granular cracking occurred in the presence of a tensile stress (but not otherwise).

The cause was traced to polythionic acids (H 2 S x O 6 where x = 3,4,5) formed by reaction of residual metallic sulfi de fi lms on the equipment surface with moist air at room temperature [63 – 66] . Such acids can be produced in the laboratory

by bubbling H 2 S through water saturated with SO 2 . It is important to the mecha- nism of failure that these acids are readily reduced cathodically by operating corrosion cells.

One function of polythionic acids is to act as cathodic depolarizers, thereby stimulating dissolution of chromium - depleted grain - boundary material. Another possible function is that their cathodic reduction products (H 2 S or analogous compounds) stimulate absorption to interstitial hydrogen by chromium - depleted

360 ALLOYING FOR CORROSION RESISTANCE; STAINLESS STEELS

alloy. The latter alloy along grain boundaries, ferritic in structure, is subject to hydrogen cracking when stressed, unlike the grains austenitic in structure that

are resistant. The function of > 2 ppm sulfur as Na 2 S or as cathodic reduction products of sulfi tes ( SO 2 3 − ) or of thiosulfates ( SO 2 2 − 3 ) to induce hydrogen cracking of a 0.77% C high - strength steel and also of ferritic and martensitic stainless steels was demonstrated in seawater [67] . It is expected that polythionic acids would act in an analogous manner.

The mechanism of failure has been described as S.C.C. by electrochemical dissolution along an active path exposed through application of stress [68] . Alter- natively, the mechanism may be one of progressive hydrogen cracking along the grain boundaries of sensitized alloy. An acid environment is necessary to failure because reaction with alloy supplies the required hydrogen; it also favors forma-

tion of H 2 S (rather than HS − or S 2 − ), which is the primary catalyst poison stimu- lating absorption of atomic hydrogen by the alloy. Aqueous SO 2 solutions are also found to cause intergranular cracking of sensitized 18 − 8, similar to that by polythionic acids, because SO 2 3 − is readily reduced at cathodic sites forming H 2 S or similarly acting reduction products, whereas sulfuric acid is much less effective in causing cracking because SO 2 4 − is not similarly reduced. Cracking in polythionic acids is most pronounced in the potential range

0.04 – 0.34 V (S.H.E.) [68] . This potential range lies above the values associated with hydrogen ion discharge and would seem to rule out hydrogen cracking. However, the relevant potential is not the one measured at the alloy surface, but the one prevailing at the crack tip, which can be appreciably more active. In practice, use of stabilized stainless steels avoids intergranular cracking of the kind described.

To Reduce or Eliminate Cracking

Austenitic Stainless Steels

a. Cathodically protect. The critical potential of 18 – 8 stainless steel in MgCl 2 at 130 ° C is − 0.128 V (S.H.E.). Coupling of stressed 18 – 8 to a small area of nickel ( φ corr − 0.18 V) prevents cracking in this medium, or (as a porous =

nickel coating) in water containing 50 ppm Cl − at 300 ° C [69] .

b. In acid environments, eliminate Cl − . In neutral or slightly alkaline chloride environments, eliminate dissolved oxygen and other oxidizing ions. Add

extraneous ions (e.g., NO − 3 ,I − , acetates).

c. Avoid high concentrations of OH − . High concentrations of alkalies occur- ring initially or incidentally through concentration at crevices or in vapor zones are damaging. Add buffering ions (e.g., PO 3 4 − ).

d. Use alloys containing > 45% Ni or reduce the nitrogen content (and other detrimental impurities, if present) to the lowest possible value ( < 0.04% N in the case of 20% Cr, 20% Ni, 0.001% C stainless steel) (Fig. 19.10 ).

e. Substitute ferritic alloys (e.g., type 430 or controlled purity Cr – Mo steels). However, ferritic alloys may become embrittled by hydrogen or they may

STAINLESS STEELS

blister when galvanically coupled to more active metals in certain environments.

f. Operate below 60 – 80 ° C (140 – 175 ° F).

Martensitic, Precipitation - Hardening, or Ferritic Stainless Steels

a. Avoid excess current if cathodic protection is applied.

b. Avoid galvanic coupling to more active metals.

c. Temper martensitic or precipitation - hardening steels to the lowest possi- ble hardness values. Exposed to the atmosphere, hardness should be below about Rockwell C 40. Maximum susceptibility of types 410 and 420 stainless steels to cracking in salt spray or to hydrogen cracking occurs after tempering for 2 h at 425 – 550 ° C (800 – 1000 ° F); minimum susceptibil- ity to hydrogen cracking occurs after tempering for 2 h at 260 ° C (500 ° F)