While in the process of trying to submit an article about corrosion protection, I reveiced a very strong comment by a referee that states
" stainless steel is passive and it cannot be protected by organic protective"
While considering that the coating I studied was an hybrid (organic-inorganic), I'm quite puzzled by the hasty comment, since I of course reported in the intro a lot of literature where stainless steel is protected by organic coatings. So very frankly what do you think about it ?
Whether stainless steel can be effectively protected by a coating depends upon the grade of stainless, the environment and the expected mode of failure. I have seen situations where 316 stainless was used in a marine atmosphere where 316 is subject to crevice corrosion and pitting. The stainless pipe was painted with an epoxy paint, which was effective in that it prevented the marine salt atmosphere from contacting the susceptible locations on the pipe. Any significant holidays in the coating would still have had the potential for pitting corrosion, but these would have been readily apparent due to the rust staining and could be quickly recoated.
However, if this application had been running at a temperature over 65C where the 316 would have been susceptible to chloride stress corrosion cracking, the coating would not have been very effective in preventing cracking since that mode of failure would occur quite rapidly at any defects in the coating.
Like Stephen already mentioned, protection through organic/inorganic/hybrid coatings depends on various factors and barrier strength is one of those provided that there is no stress corrosion cracking. The surfaces that can produce active surface functionalities (i.e., OH etc.) tends to form much stronger bonds with the coatings and have less tendency of delamination (in the event when electrolyte passes through the existing holidays in barrier layers). However, passive surface don't have enough surfaces functionalities for the coating to construct stronger bonds, tend to delaminate easily. Mechanical anchoring of coating (i.e, by roughing the surface) promotes the adhesion in such cases, but the barrier layer thickness (with condensed network) should be high enough to act as impervious/ shield for the incoming or percolating electrolyte. The presence of interconnected microvoids through the coating to the surface of metal will generate localized regime for accelerated corrosion and severe pitting.
Having said that, protection through coatings is always relative (phenomenon) to the available state of the art and any blanket will provide protection to some extent. Protection from your coating may not meet the established industrial standards but if it is adhering to the metal surface, it will protect it for some time for sure :)
Greetings Dr. Giorgio Luciano ,
I am trying to develop the same type of organic inorganic coating as in your question and test it in seawater. Can you suggest the coating thickness which is suitable for this type of situation where we want to test it on SS and immersed it seawater?
And if not a problem, can you educate me if that suggestion is related to any standard such as ASTM.
Thank you.
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