HEAT-RESISTANT SILICONE COATING
FOR THE CONTROL OF
EXTERNAL STRESS CORROSION CRACKING (ESCC)
OF STAINLESS STEEL
PREVENTION OF EXTERNAL STRESS CORROSION CRACKING OF STAINLESS STEEL
“ESCC (External stress corrosion cracking) occurs when (1) stresses austenitic stainless steel is present, (2) there is a hot or warm metal surface and (3) when a crevice or joint exists where chlorides and moisture can be trapped. Under these conditions, the wet chlorides from sea air or process vapors will concentrate on the hot metal, reach a threshold value, and stress cracking will initiate. This situation can occur under insulation flanges, gaskets, pipe clamps bolt threads and many other areas.
“One unusual situation for the ESCC is found on stainless steel rupture discs. Every so often one of them blows out without apparent reason. Here chlorides from the atmosphere settled in the crevice between the rupture disc and steel retaining flange, heat from the process concentrated the salts and failure occurred.
“Because the chloride ion is ever present, we feel it is impractical to build and operate stainless steel equipment in a chloride-free environment. Based on a series of tests, we concluded that a coat of paint is the most practical way to protect the equipment. This paint program has been in effect for more than eight years with no incidents of ESCC on any protected equipment.
“Not all chlorides are in the air however. Figure 4 shows what happens when a chloride-containing liquid drips out of a valve packing onto a hot stainless steel line. The organic chloride was inside this same line with no problems, but upon exposure to air and heat, HCL was generated and rapid attack occurred.”
The coating referred to above is Thurmalox 70.
Some very significant points to consider relating to applications of Thurmalox 70 and equipment design features are contained in these articles. Subcontractors supplying unitized subassemblies should be apprised of the vulnerable nature of flanges, bolts threads, etc. Such surfaces should be coated at the time subassembly and again at the time of final erection.
There are many theories concerning the cause and effect relationship to stress cracking of stainless steel and several papers have been published on the subject. A
selection of these are enclosed as a part of this brochure. It appears that a major role of thermal insulation is that of trapping chloride contamination in place under insulation. Such being the case, over a period of time concentration of chlorides can rise until it reaches the danger level.
Cracking of stainless steel can also be caused by molten zinc and other metals. An explanation of this phenomena is contained in the attached reprints entitled “Bi-Weekly,” by C. P. Dillion of Union Carbide Corporation. From this it can be inferred that precautions should be taken to eliminate the possibility of zinc and other metals from coming into contact with stainless steel in addition to eliminating the possibility of contact with chlorides. There are also some recently identified cracking problems relating to the presence of sulfides and nitrates. Inconel and/or cupronickel alloys have been the victims in these cases.
From the foregoing it follows that due precautions should also be taken to see that protective coatings applied to stainless steel prevent chloride stress cracking are formulated to contain only minimum practical amounts attainable of metals and/or halides, sulfides, and nitrates which of themselves could trigger such stress corrosion failure. In particular, zinc rich coatings should never be permitted to contact stainless steels.
Thurmalox 70 is formulated to be as practically free of the aforementioned potentially hazardous contaminants as possible. A typical analytical report describing the levels of trace elements present in Thurmalox 70 is attached. The determinations were made by National Spectrographic Laboratories, of Cleveland, Ohio. The heavy metals were determined by the atomic absorption procedure while the halides and sulfur were determined by the Specific Ion Electrode procedure. Such certified analytical reports can be made available to those requiring them for each batch of Thurmalox 70 produced.
Questions also arise concerning data or experience relating to external stress cracking of other alloys. It appears that there is no alloy completely immune. It merely remains for the right circumstances to occur (chemical environment, temperature and mechanical stress).
Field experience accumulated over a twelve (12) year period has demonstrated that applications of Thurmalox 70 can effectively prevent external stress corrosion cracking of stainless steels. To the best of our knowledge there has never been as external stress cracking failure where Thurmalox 70 has been applied.
For further information of the subject of stress corrosion cracking we refer you to:
Dampney Company Inc.
85 Paris St Everett, MA 02149