Stress corrosion cracking chlorides

Materials of Construction. GeneraHy, carbon steel is satisfactory as a material of construction when handling propylene, chlorine, HCl, and chlorinated hydrocarbons at low temperatures (below 100°C) in the absence of water. Nickel-based aHoys are chiefly used in the reaction area where resistance to chlorine and HCl at elevated temperatures is required (39). Elastomer-lined equipment, usuaHy PTFE or Kynar, is typicaHy used when water and HCl or chlorine are present together, such as adsorption of HCl in water, since corrosion of most metals is excessive. Stainless steels are to be avoided in locations exposed to inorganic chlorides, as stainless steels can be subject to chloride stress-corrosion cracking. Contact with aluminum should be avoided under aH circumstances because of potential undesirable reactivity problems. 

Remedial measures include reduction or elimination of chlorides or replacement of the 304 stainless steel with a metal that is resistant to chloride stress-corrosion cracking. 

The corrosive environments which cause SCC in any material are fairly specific, and the more common combinations are listed in Table 53.2. In the case of chloride stress corrosion cracking of the 530 series austenitic stainless steels it is generally considered that the risk is... 

Boiler salts can contain chloride ions. When carried over into the steam (e.g. during priming) this can result in chloride stress corrosion cracking of austenitic stainless steel expansion bellows. In steam systems where freedom from chloride cannot be guaranteed, bellows... 

D ye penetration inspection. This is a simple technique, requiring a minimum of operator training. In the hands of a skilled operator, it is capable of detecting fine cracks such as chloride stress corrosion cracks in austenitic stainless steels and fatigue cracks. 

Rhodes, P. R., Mechanism of Chloride Stress-corrosion Cracking of Austenitic Stainless Steel , Corrosion, 25, 462 (1969)... 

Where, for example, chloride stress-corrosion cracking is a risk the process temperature becomes a critical variable. Thus it may be more economic to lower the process temperature to below 70°C, a practical threshold for chloride stress-corrosion cracking, than to incur the extra expense of using stress-corrosion cracking-resistant materials of construction. 

Alloys of high nickel content also have improved chloride stress-corrosion cracking resistance and Incolloy 825 has replaced type 321 stainless steel for steam bellows on some plants. Occasionally cracking of the latter was experienced due to chloride-contaminated steam condensing in the convolutions on shut-down and being re-evaporated at start-up. 

To protect stainless-steel equipment from chloride stress-corrosion cracking by triggering an anodic protection system when the measured potential falls to a value close to that known to correspond to stress-corroding conditions. 

The use of hydroxyacetic/formic acid in the chemical cleaning of utility boilers is common. It is used in boilers containing austenitic steels because its low chloride content prevents possible chloride stress corrosion cracking of the austenitic-type alloys. It has also found extensive use in the cleaning operations for once-through supercritical boilers. Hydroxyacetic/formic acid has chelation properties and a high iron pick-up capability thus it is used on high iron content systems. It is not effective on hardness scales. 

The chloride stress-corrosion cracking of austenitic stainless steels in chloride solutions with samples under tensile stresses has been known since 1940. There are some reports that claim the retained austenitic structure to be responsible for chloride stress-corrosion cracking. 

The duplex stainless steels are superior to other stainless steels with respect to high resistance to chloride stress corrosion cracking, high mechanical strength, lower thermal expansion than the austenitic grade steels, and good erosion and wear resistance. 

The third example describes mistakes made during the design, procurement and construction of a high-pressure steam piping system, resulting in a catastrophic failure of a pipe connector, due to chloride stress corrosion cracking. 

The pipe joint had been leaking steam and water prior to the failure, and chemical analysis of the scale deposits on the clamp surface after the failure confirmed the presence of a number of sodium-based mineral compounds from the leaking steam, including approximately 10% sodium chloride. The presence of high concentrations of moist, hot chloride salts on the highly stressed austenitic stainless steel surface, particularly with concurrent exposure to atmospheric oxygen, created an ideal chloride stress-corrosion cracking (SCC) environment. 

There were also concerns about the potential for chloride stress corrosion cracking with the materials used in the HGCU. 

Four hundred-series alloys are resistant to chloride stress corrosion cracking, but not chloride pitting. Accordingly, they are rarely used in aqueous chloride services. However, superferritic ... 

With the desired microstructure, these alloys are resistant to hydrogen stress cracking and much more resistant to chloride stress corrosion cracking than are the austenitic stainless steels. (The threshold temperature for chloride stress corrosion cracking of duplex alloys in neutral pH aqueous chlorides is about 300°F [150°C].) The chloride stress corrosion cracking resistance of the duplex alloys is similar to that of superaustenitic alloys such as Alloy AL-6XN. Because they contain about 50% ferrite, the duplex stainless steels are more susceptible to hydrogen embrittlement. 

Washing with an inhibited acid to dissolve the oxides and sulphides of iron. Hydrochloric acid was not used because of the risk of chloride stress corrosion cracking of the stainless steel. Sulphuric acid was used. 

Strictly speaking, chloride stress-corrosion cracking will not occur unless there is contact with an aqueous solution of suitable chloride concentration, a favorable temperature and strain or residual stress. These requirements may, however, be met rather unpredictably. 

Figure 5.70 Chloride stress corrosion cracking in stainless steel. (Figure originally published in Reference 26. Reproduced with permission of the Canadian Institute of Mining, Metallurgy and Petroleum, www.cim.org.)...

R.K. Singh Raman, W.H. Slew, Role of nitrite addition in chloride stress corrosion cracking of a super duplex stainless steel, Corros. Sci. 52 (2010) 113—117. 

K. Tamaki, S. Tsujikawa, Y. Hisamatsu, Development of a new test method for chloride stress corrosion cracking of stainless steels in dilute NaCl solutions, in H.S. Isaacs, U. Bertocci, J. Kruger, S. Stnialowska (Eds.), Advances in Localized Corrosion, in NACE-9NACE, Houston, TX, 1991, pp. 207-214. 

In Table 10.7 a comparison is made between different steels as regards the resistance to localized corrosion in a chloride environment. The resistance to chloride stress corrosion cracking is low for steels with 8-9% Ni, but it is improved strongly when the Ni content is increased further. 

R. Javaherdashti, R.K. Raman Singh, C. Panter, E.V.P. Pereloma. Role of microbiological environment in chloride stress corrosion cracking of steels. Materials Science and Technology, Vol. 21. No. 9, pp. 1094-1098, 2005. 

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