The kinetics of the reaction of steel with pure hydrogen sulfide at 250~176 may be interpreted in terms of consecutive linear and parabolic rates. In hydrogen sulfide-hydrogen mixtures similar reactions exist, but the reaction rate decreases as the thermodynamic equilibrium line (Fe ~-H~S ~--FeS + HD is approached, in a manner which is approximately proportional to the decrease in the thermodynamic driving force. Increase of pressure up to 20 atmospheres increases the reaction rate by a fractional power of the pressure. In the presence of traces of oxygen, the linear rate component is minimized. The observed kinetics can be explained in terms of three steps: adsorption, rate of formation of diffusing species, and diffusion. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 152.14.136.77 Downloaded on 2015-04-12 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 152.14.136.77 Downloaded on 2015-04-12 to IP Vol. 105, No. 4 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 152.14.136.77 Downloaded on 2015-04-12 to IP

Intergranular cracking tendencies of fourteen stainless steels and two high-nickel alloys have been evaluated by exposing numerous specimens to polythionic acid and standard acid copper sulfate solutions. The sensitization curves for solution-annealed wrought stainless steels, which had been held for varying times at temperatures from 800 to 1700 F (427 to 927 C), had the same lower temperature limit for both solutions. The higher temperature limit for acid copper sulfate was significantly higher than for polythionic acid so the more standard acid copper sulfate test can be used for control. Decreasing the carbon content shifted the sensitization curve to longer times. Times in excess of 1000 to 4000 hours were required to sensitize steels stabilized with columbium or titanium. Molybdenum additions decrease the sensitization range, but the higher-carbon molybdenum-bearing stainless steels had an unexplained sensitization zone from 900 to 1000 F (482 to 538 C) for holding times exceeding 200 hours. The 25 percent Cr-Ni materials became sensitized over a narrower range of temperature than the plain 18 percent Cr-Ni alloys and, for a given carbon content, were more stable. If 1200 F (649 C) sensitizing followed thermal stabilization, the low-carbon Type 310 and Types 347 and 321 steels were sensitized the least. Two high-nickel alloys and a cast stainless steel failed when sensitized for 4 hours at 1200 F.

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The solubility product for niobium and carbon in a Type 347 steel is Niobium additions to an 18Cr:8Ni type matrix reduce carbon solubility to such an extent that any “solution treatment” below at least 1150 C causes stabilization. Consequently, no Cr2 3C6 precipitates at lower temperatures to sensitize the structure. Further observations on Type 321 suggest that two types of TiC precipitate from solid solution in an 18:8 type matrix. The size of the TiC nucleus decreases with the precipitating temperature. Above 1050 to 1100 C the initial TiC is probably incoherent, large enough to be stable and resistant to ferric sulfate-sulfuric acid solution. Below 1050 to 1100 C the initial TiC, known as “dot TiC” or “TiC on dislocations”, is probably coherent, not large enough to be stable without further growth, and not resistant to ferric sulfate-sulfuric acid solution. During holding at temperatures below 1050 to 1100 C, stabilization occurs as the TiC on dislocations agglomerates to larger, incoherent particles. The time required increases as the temperature decreases down to the minimum TiC nucleation temperature near 610 C. Cold work makes it easier for the coherent particles to become incoherent, in effect facilitating approach to equilibrium carbon solubility at any temperature. Once chromium carbide forms, dissolved titanium eventually reacts with it, forming TiC and releasing chromium to desensitize the structure. This reaction can occur, given sufficient time for titanium diffusion, at any temperature at which chromium carbide nucleates. It is much more rapid than back diffusion of chromium from the matrix.

Selection of steels for petroleum refinery vessels and piping is determined by the most economical combination of Code allowable stresses and ability to resist service deterioration under operating conditions. Temperature is a major factor in selection because at itl effect:s_ both strength and corrosion rate;~efinery operating temperatures may range from -50~F or lower to 1200°F or higher depending on the process. HYdrogen deteriorates some steels over much of th~s t.l,e tJ/ aitl!JCIC varle5 LJ,tlc.."'I'E' a I iii e range but diCE.l _Ioct mechanism, eel r ee$;." at. hi gh temperature. Design stresses for many steels are constant in the range -20°Fto 650 00 F, but at and below atmospheric temperatures ferritic steels probllJn11f't ./ tkis haztll'Yl. may undergo a notch-brittle transitionjtheAMI Be 5 "e !6!hi wll must be recognized. fle ... " iL;Ql. xgslsb 66116310£1 pOSIts ill ~h8 Bf!tl!:;:S '-22 (J, f,'iP pse or filrni"g='ilU OJ.ISSEil izdl11d!tblll ii., le;a.h,ephg )]i'tFrom 650 00 F to 1200°F, ferritic steels containing up to 9 per cent chromium and 1 per cent molybdenum resist most forms of deterioration successfully. The higher chromium ferritic steels have little

Selection of steels for petroleum refinery vessels and piping is determined by the most economical combination of code allowable stresses and ability to resist service deterioration under operating conditions. Temperature is a major factor in selection because it effects both strength and corrosion rate; refinery operating temperatures may range from -50 degrees F or lower to 1200 degrees F or higher depending on the process. Hydrogen deteriorates some steels over much of this range but the mechanism of attack varies with temperature. Design stresses for many steels are constant in the range -20 degrees F to 650 degrees F, but at and below atmospheric temperatures ferritic steels may undergo a notch-brittle transition; the probability of this hazard must be recognized. From 650 degrees F to 1200 degrees F, ferritic steels containing up to 9 percent chromium and 1 percent molybdenum resist most forms of deterioration successfully. The higher chromium ferritic steels have little merit. Above 1200 degrees F, and for severely corrosive conditions at lower temperatures, austenitic stainless steels must be used.