I look for suggestions in clarifying a corrosion issue related to some equipment components, made of superduplex S32760.
We were required to perform the final cleaning of the steel according to the provisions of ASTM A380, acceptance test acc. to paragraph 7.3.4, using Ferroxyl solution test.
After several pickling stages the steel still gets some blue points, which seems not acceptable. This happens within a few minutes and not 15 seconds as indicated by the provisions of paragraph 7.3.4.
Moreover tests on small mirror polished specimens, done in a separate laboratory to avoid any possible contamination, gave negative results: small blue coloured areas still appear.
We think that Ferroxyl solution test is not applicable to other stainless steel than austenitic and assume that that the ferrite content of superduplex is at the origin of the blue colour, not the carbon steel contamination.
The normal procedure to be followed would be according to A380 paragraph 7.2.5.3, using copper sulfate solution, which returns acceptable results in our case.
We were required to perform the final cleaning of the steel according to the provisions of ASTM A380, acceptance test acc. to paragraph 7.3.4, using Ferroxyl solution test.
After several pickling stages the steel still gets some blue points, which seems not acceptable. This happens within a few minutes and not 15 seconds as indicated by the provisions of paragraph 7.3.4.
Moreover tests on small mirror polished specimens, done in a separate laboratory to avoid any possible contamination, gave negative results: small blue coloured areas still appear.
We think that Ferroxyl solution test is not applicable to other stainless steel than austenitic and assume that that the ferrite content of superduplex is at the origin of the blue colour, not the carbon steel contamination.
The normal procedure to be followed would be according to A380 paragraph 7.2.5.3, using copper sulfate solution, which returns acceptable results in our case.
