A new method for assessment of susceptibility to chloride stress corrosion cracking under severe evaporative conditions has been developed. The basic idea is to test under submerged conditions simulating the electrolyte composition and concentration of sea water during evaporation. Two duplex and one austenitic stainless steel were tested loaded to the yield point at the test temperature, Time to failure, potential and temperature were recorded continuously.
The results showed cracking of the austenitic material UNS S31254 at 110ºC by long term testing. For the duplex stainless steels UNS S31803 and UNS S32750 cracking was observed at 100ºC, The time to cracking was longest for the last mentioned and highest alloyed duplex material.
In marine environment e.g. above sea level on offshore installations, in spray zones on boats and in coastal atmospheres in petrochemical plants, hot process piping systems are exposed to a severe environmental condition. The electrolyte on the wetted surfaces will, when the water evaporates, turn into highly concentrated chloride solutions. Since near to dryness evaporation leads to a very small volume of electrolyte, the risk of severe localized corrosion in form of perforating pits and crevice corrosion is likely to be small in most practical situations, Some staining and superficial pits may be observed, but not with any significant depth in the corrosion attack, The situation may be different if the electrolyte is trapped under insulation. In this case ratio between the passive areas and the corroding areas maybe large and hence, the local corrosion rate high.
Stress corrosion cracking of stainless steels under evaporative conditions has for long been recognized as a factor that needed special attention since the propagation rate of SCC can be marry times the propagation rate of a pit. This means that failure due to SCC can occur within very short periods of time,that is within a few days or even hours in the most extreme cases.
In spite of the fact that chloride stress corrosion cracking under evaporative conditions is recognized as a severe problem, data reflecting the suitability of the different stainless steels under these conditions are only to a small extent available, This is in particular the case for the new highly alloyed stainless steels.
The aim of this project was developing a new test method that could produce data which was able to give a statement with respect to temperature limits for chloride SCC under the most severe evaporative conditions occurring on an offshore platform.
In he first part of this work chloride stress corrosion cracking of the materials UNS S31803, LJNS S32760 and UNS S31254 has been studied by the Drop Evaporation Test1, The method was, however, not suitable for long term testing when sea water was used as electrolyte. It was therefore decided to test under submerged condition simulating the electrolyte composition and concentration of sea water during evaporation, Results from the tests of two duplex stainless steels UNS S32750, UNS S31803 and one austenitic stainless steel UNS S31254 are reported in this paper.
