Richard J Pargeter
Paper IPC-98-170 presented at the Second International Pipeline Conference (IPC'98), Calgary, Alberta, Canada, 9-11 June 1998.
Published data on the effects of different levels of hydrogen sulphide (H2S) and pH in aqueous environment environments on steels have been reviewed. Both sulphide stress corrosion cracking (SSCC) and hydrogen pressure induced cracking (HPIC) have been considered. The data have been collated and presented on one diagram, and the appropriateness of setting relaxed hardness controls for 'mildly sour' environmental conditions has been discussed.
Wet sour environments are particularly aggressive to steels, because sulphide scales 'poison' the atomic hydrogen combination reaction, and thereby encourage hydrogen generated by corrosion reactions to be absorbed by the steel. Hydrogen atoms embrittle steel at around normal ambient temperatures, and, in the presence of stress, may cause cracking. This is the mechanism by which sulphide stress corrosion cracking (SSCC) occurs, and it is commonly controlled in industry by setting limits on hardness (which gives a measure of the susceptibility of the steel to embrittlement) and on environmental severity (which affects the amount of hydrogen which the steel may absorb). Historically, requirements have set one limiting hardness, in conjunction with a limiting H2S level, [1,2] despite the fact that safe hardness level and environmental severity are interdependent. Furthermore, no account has usually been taken of other environmental factors, such as pH. The advantage of the traditional approach is simplicity, but where, for example, it comes to light at a late stage in fabrication that controls have not been adequate, and hardness levels are marginally above specification, or equipment which was not originally designed for sour service is required to handle a sour environment, there may be a need to consider the effects of 'mildly sour' environments in more detail. More recent specifications have begun to explore the boundaries of sour definitions, and hardness limits, often leading to apparent relaxations in conservatism  . It is therefore pertinent to review the current state of knowledge concerning steel behaviour in 'mildly sour' environments.
A second problem which sour service can cause in rolled steels, namely hydrogen pressure induced cracking (otherwise variously known as hydrogen-induced cracking (HIC), stepwise cracking (SWC), blister cracking, and by other terms)(HPIC), has also been shown to be dependent on hydrogen concentration within the steel , with different steels having different threshold hydrogen concentrations for cracking (Cth). A cracking mechanism which has not been discussed in this paper is Stress Oriented Hydrogen Induced Cracking (SOHIC). This occurs only under severe conditions, and this is of no concern in 'mildly sour' service.
In this paper, published data have been reviewed and collated, taking due account of test method, specimen type, and applied stress levels. The data have been presented on one plot of pH vs HsS partial pressure. This is used to consider the validity of the concept of mildly sour conditions, by comparison with older definitions of sour service.
Previous explorations of H2S level
Sulphide stress corrosion cracking
In 1966, Hudgins et al were proposing that, rather than a distinction between 'sweet' and 'sour' conditions, there may only be varying degrees of 'sourness'  . Their concern was that there was no truly safe 'sweet' region, but equally they indicated an increase in safe hardness level as H2S content fell. Their compilation of data ( Fig.1) does not take pH into account, but the effects of both H2S content and pH were considered by Dvoracek in 1970  , for P-110 steels, using pre-cracked specimens. This work clearly indicates that safe stress levels are higher at higher pH and lower H2S levels ( Fig.2).
Fig.1. Approximate correlation of failure time to hardness in 5 percent NaCl containing various concentrations of H2S. After Hudgins et al 
Fig.2. Effect of H2S and pH on the sulphide stress corrosion cracking susceptibility of API Grade P-110 steel, quenched and tempered, 132 ksi yield strength, 75F (24C). (1) 25 ppm H2S, (2) 150 ppm H2S, (3) 300ppm H2S, (4) 2800ppm H2S. After Dvoracek 
1981, Japanese workers from Kawasaki Steel Corporation (KSC) and NKK presented data on the effect H2S concentrations. Both sets of workers used four point bend specimens, which were exposed to 0.5% acetic acid solutions with different H2S concentrations. NKK  stressed their specimens to 1.3% yield strain, whereas the stress level is not stated in the RSC report.  The results ( Fig.3 & 4) are similar, and clearly indicate that higher hardness can be tolerated at lower H2S levels. On the basis of their work, NKK recommended four zones of H2S partial pressure, as shown in Fig.5.  Zone A is equivalent to below the current NACE limit,  0.05psia H2S, and no special precautions are required. In Zone B a hardness limit of 260HV, (and a Ni restriction of <1.0%) is imposed. NACE MR0175 precautions (including a 248HV limit) are advised for zone C, but with(unspecified) reductions in Ni and sulphur levels. For zone D, more severe limits are advised, but no values are given. In work apparently leading up to these proposals, an equation relating critical hardness to H2S partial pressure is proposed by NKK  : Critical HV10 = 395 - 60log x, where x = H2S concentration in water (ppm). It is questionable, however, whether this is applicable to mixed H2S/CO2 environments, or other situations where pH is controlled by species other than H2S.
Fig.3. Effect of hardness and H2S concentration on SSCC. After Motoda and Yamane. 
Fig.4. Critical hardness for SSC in girth welded joints relating to H2S partial pressure. Steel: Grade X52/X65, Stress level: D = 1.3 •Dys, Dys: yield deflection. After Taira et al. 
Fig.5. Classification of service condition for sour gas pipelines. After NKK. 
Prior work was reviewed by Gooch  in 1982, and he was able to produce a plot of H2S content vs HAZ hardness (Fig.6), which indicated an increase in threshold hardness for cracking from about 240HV at 3000ppm H2S in solution (saturation at 1 bar) to around 350HV at 10ppm H2S in solution. Where pH had been reported, this was annotated on the plot, and this suggested an effect of pH. It should also be recognised that, for the rest of the data points, there will, in general, be some trend towards lower pH levels at higher H2S contents, as H2S is in itself an acid gas.
Fig.6. Relationship between H2S concentration in aqueous solutions and maximum permissible HAZ hardness. Derived from tests on as-welded pipeline and structural steels, stressed to 100 - 130% parent yield stress, data points being taken as defining limiting conditions. After Gooch. 
Workers from Elf and BP defined safe operating conditions for a range of down-hole tubular materials.  An N80 13%Cr steel, and a P110 carbon steel, with hardness of 263 and 294HV respectively, defined the boundaries of 'sour', 'non-sour' and 'transition' regions, incorporated in EFC document 16  ( Fig.7). Again, this demonstrates a combined effect of pH and H2S content.
Fig.7. Sulphide stress corrosion cracking domains as a function of pH and hydrogen sulphide partial pressure. After Kermani et al. 
In summary, there have been several individual attempts to explore hardness limits appropriate to 'mildly sour' conditions at least since the 1960s. It is clear that there is a combined effect of pH and H2S and that useful relaxations in permissible hardness are possible as the H2S activity is reduced or pH is increased.
Hydrogen pressure induced cracking
The situation with regard to HPIC was reviewed by Biefer in 1982.  Overall, it is evident that the same combined effects of pH and H2S occur as for SSCC, but that threshold conditions for onset of cracking, even in very susceptible steel, are an order of magnitude more severe than those generally considered for SSCC. Threshold H2S partial pressures of around 1 psi (compared with the general NACE MR0175 limit of 0.05psia) are generally quoted.
From large scale tests, Ikeda et al determined that the critical partial pressure of H2S for the initiation of HPIC is between 0.06bar and 0.35bar,  although it is not entirely clear how this boundary was derived, nor what other components of the environment were. In later work,  Ikeda et al presented a diagram (Fig.8) in which the combined effects of CO2 and H2S were shown. Relating this to tabulated information on tests steels indicates that, below about 0.1bar H2S, even a highly susceptible steel would be at little risk of cracking.
Fig.8. Influence of the partial pressure of H2S and CO2 on HPIC in steels with different Cth values. After Ikeda et al. 
In a report on linepipe steels for sour service,  Nippon Steel Corporation divide an H2S vs CO2 partial pressure plot into four regions ( Fig.9). Below about 0.1bar H2S, HPIC is not considered to be a problem.
Fig.9. Corrosion behaviour in terms of partial pressure of CO2 and H2S in synthetic sea water at between ~ 45 & 60°C • : examined condition in experimental study. After work by Nippon Steel Corporation. 
Overall summary of published data
In this section, published data pertaining to SSCC in mildly sour environments have been collected together. These cover a range of test methods and environments. Hardness values are as published, but very often assumptions or calculations of pH have had to be made. The data were as follows:-
- Three data points taken from work by Pargeter et al  covering testing at ambient pressure in 5%NaCl and ASTM D1141-75 standard substitute ocean water. Values of pH and H2S were measured directly. Test specimens consisted of transverse weld three point bend samples, approximately 25mm square cross section, with the weld cap remaining intact. The thresholds quoted are for applied yield stress in the outer fibres, not taking stress concentrating effects of the weld cap into account.
- Five data points taken from a study by Motoda and Yamane.  Smooth four point beam specimens were stressed to yield in 0.5% acetic acid solution with various H2S contents. A pH of 3.5 has been assumed. The hardness thresholds are taken from the figure presented in the paper, but may be unconservative by comparison with other data as some specimens containing 'microcracking' around 50µm deep were deemed uncracked.
- Five data points taken from work by Nakazawa and Tanimura  in which they tested welds in distilled water containing various levels of H2S. Values of pH for these environments have been calculated using CORMED. 
- Two points from Taira et al  who stressed smooth, 3mm thick, four point bend specimens machined from welds to 1.3 x yield deflection in 0.5% acetic acid solution. As for Motoda and Yamane's work above, a pH of 3.5 has been assumed. The failure criterion was not specified, but the loading was severe, so the data are unlikely to be unconservative, and indeed are more conservative than Motoda and Yamane's.
- Three data points from Vennett  who tested 3mm thick transverse samples with the weld cap intact in three point bend in 3%NaCl solution with a range of pH and H2S levels. Stressing by deflection was to nominally very high outer fibre stresses, well in excess of parent material yield, and crack detection was by metallographic examination. These data can be taken as relatively conservative.
- Nine points from Kermani et al.  These are the data points on the basis of which the boundary lines for sour service regimes were drawn, as shown in Fig.7, which are now incorporated in EFC document 16.  The results are from smooth tensile tests carried out on N80 13%Cr steel and P110 carbon steel, with reported hardnesses of 263 and 294 HV respectively.
- Three data points from Nisbet et al.  Two points arise from a full scale test at 140 bar total pressure for which the pH was calculated to be 4.7-5. With 26mbar H2S, bead on plate and weld repairs with a total range of hardness down to 322 HV cracked. As the hardnesses of these two welds also went up to 366 and 360 HV respectively, 322 HV is a conservative value to choose. With 7mbar H2S neither of these welds cracked, so the threshold is >366 HV. The third data point is from smooth tensile tests at 90% actual yield stress with 7mbar H2S and 1 bar CO2, pH 4.7-5.3, where cracking was experienced above 340 HV.
- Two data points from work reported by McIntyre and Boah.  Welds made in A516 grade 70 steel were tested under low pH/H2S combinations to determine the threshold hardness for each.
The data above have been plotted on a pH vs H2S diagram as shown in Fig.10, with the NACE MR0175 limit and boundaries proposed in EFC document 16  superimposed.
Fig.10. Summary of data on SSCC in 'mildly sour' environments reported in the literature. Threshold hardness and reference no. given by each datum point
The sour service definitions proposed by EFC document 16  were developed for oil country tubular goods, and are strictly only applicable to such materials, as indeed stated in Section 7.2.1 of that document. Nevertheless, it is also applied to welded steels in Table 8.1. Using the wider database presented in Fig.  , this approach is broadly supported. However, it is apparent that some hardness control is necessary in the EFC 'non-sour domain',  and it is arguable that the boundary lines are not ideally placed.
In Table 8.1 of EFC document 16,  a hardness limit for welds of 260 HV is given for the transition domain between the two lines in Fig.10. Although no data points below 260HV actually fall within this region, the lower bound is apparently not very conservatively placed. With regard to the 'non-sour' region above both lines (see Fig.7), but still above the NACE MR0175 limit, thresholds of between about 270 and 370 HV are reported below pH 6.5. Indeed, the one data point below the NACE MR0175 limit suggests that even in this traditionally sweet regime, some hardness control is needed. The EFC document is not clear on safe weld hardness levels in the 'non-sour' domain, only giving guidance on 'sour' and 'transition' domains.
To redefine the position of the lower boundary line, more data are necessary, but some of the points on the line suggest that it should be placed slightly more conservatively to the left. If the hardness limit in the current non-sour domain, above both lines, is to be above 300 HV, then the upper line would need to be moved to encompass the two data points from ref.  , and that from ref.  . In that case, on present information, a limit of around 325 HV might be appropriate for the non-sour domain. Below the existing NACE MR0175 limit, a reasonable fabrication standard of around 350 HV would probably provide an adequate safety net.
The definition of sour conditions in NACE MR0175 has, in recent years, eclipsed the essentially more scientific consideration of the effects of H2S and pH referred to above. Equipment designed in accordance with MR0175 has a good track record, however, and although some economies could perhaps have been achieved by relaxing hardness limits for slightly less sour or higher pH service conditions, this would probably not have balanced the overall benefits of a very simple approach. It should also be noted that it is not always easy to measure or predict precise environmental conditions, particularly over extended times.
On occasion, however, situations arise where it is appropriate to explore the level of conservatism in such design rules. It is evident from the data surveyed that there is a fairly narrow region of H2S and pH where useful hardness relaxations can be permitted, but that within those regions the allowable relaxations can be significant. As these are relaxations from current practice, the boundaries need to be defined with some care, and reassessment of the guidance in EFC document 16 is advised.
- NACE: 'Sulfide stress cracking resistant metallic materials for Oilfield Equipment', Standard Material Requirement MR0175-97.
- NACE: 'Methods and controls to prevent in-service cracking of carbon steel welds in P-1 materials in corrosive petroleum refining environments', Standard RP0472-87.
- European Federation of Corrosion: 'Guidelines on materials requirements for carbon and low alloy steels for H2S containing environments in oil and gas production', EFC document No. 16, 1995.
- Ikeda A, Terasaki F, Takeyama M, Takeuchi I, Nara Y: 'Hydrogen induced cracking (HIC) susceptibility of various steel line pipes in the wet H2S environment', Corrosion 78, NACE, March 6-10 1978, Houston, Texas, Paper 43.
- Hudgins C M, McGlasson R L, Mehdizadeh P and Rosborough W M: 'Hydrogen sulfide cracking of carbon and alloy steels', Corrosion, 22 (8) August 1966.
- Dvoracek L M: 'Sulfide stress corrosion cracking of steels', Corrosion 26 (5) May 1970.
- Taira T, Tsijkada K, Kobayashi Y, Inagaki H and Watanabe T: 'Sulfide corrosion cracking of line pipe for sour gas service', NACE, Corrosion 37 (1), January 1981.
- Motoda K and Yamane Y: 'Stress corrosion cracking of steels in aqueous solution of low hydrogen sulfide concentration' presented at 101st ISIJ meeting, April 1981, Lecture no. S477.
- NKK: 'Sulfide corrosion cracking of welded line pipe in sour gas service'. NKK Technical Bulletin.
- Private communication from NKK, February 1977.
- Gooch T G: 'Hardness and stress corrosion cracking of ferritic steel'. Proc Seminar 'Hardness and hardness control of welded fabrications', TWI, Bradford, 1981. See also Welding Institute Research Bulletin; 23 (8), August 1982, pp.241-246.
- Kermani M B, Harrop D, Truchon M L R and Crolet J L: 'Experimental limits of sour service for tubular steels', NACE Corrosion 91, Paper 21, March 11-15, 1991.
- Biefer G J: 'The stepwise cracking of linepipe steels in sour environments', Materials Performance June 1982.
- Ikeda A, Kaneko T and Terasaki F: 'Influence of environmental conditions and metallurgical factors on hydrogen induced cracking of line pipe steel'. Corrosion 80, NACE, 3-7 March 1980, Chicago. Paper 8.
- Nippon Steel Corporation: 'Development of linepipe steels for sour gas service', NSC report on the linepipe steels for sour gas applications: Part II, report 1102 SOUR-02-81-0, February 1981.
- Pargeter R J, Gooch T G and Bailey N: 'The effect of environment on threshold hardness for hydrogen induced stress corrosion cracking of C-Mn steel welds'. Conference Proceedings 'Advanced Technology in Welding, Materials, Processing and Evaluation', Japan Welding Society, Tokyo, April 1990.
- Nakazawa T and Tanimura M: 'SSCC in welded parts of high strength steels', Nippon Kokan Technical Research Centre of Kakushiki Kaisha, October 1978.
- Crolet J-L and Bonis M R: 'An optimised procedure for corrosion testing under CO2 and H2S gas pressure', Materials Performance, 29(7) July 1990, pp.81-86.
- Vennett R M: 'Safe weld hardness levels to insure long service life in offshore pipelines', Offshore Technology Conference, May 1-3, 1972, Houston Texas, Paper OTC 1571.
- Nisbet, W J R, Leune M, Attwood P A and Robinson J L: 'The effect of local hard zones on the sulphide stress corrosion cracking of C-Mn steels exposed to mildly sour environments', Corrosion 95, Paper 69, March 26-31, 1995, NACE.
- McIntyre D R and Boah J K: 'Review of sour service definitions', Materials Performance 35 (8) August 1996, pp.54-58.
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