It is not unusual to hear many engineers lay claim that by using stainless steel piping, that they are fully eliminating the risk of corrosion under insulation (CUI) within their piping circuits. But could stainless steel piping be susceptible to CUI much like it’s cousin in the piping family… carbon steel?
The short answer is YES, even stainless steel can fall prey to corrosion in certain conditions. Here is how…
Stainless steel’s Kryptonite is “chlorides” and when we have the two together, either from the water source or the insulation selected we have aligned the planets such that we can in fact have a piping failure caused by corrosion under insulation (CUI) on our stainless steel piping circuit. I first learned about this while working for a paper company and was working a turn around in the Charleston, SC region and we found corrosion on a SS vessel during a vessel inspection. The source was the salt water humidity and fog, which was such an issue with this plant that a nearby bridge had a fog warning system installed on it to help with traffic at certain times of the year.
Technically speaking, it is called External Stress Corrosion Cracking or ESCC for yet another acronym in the world of Process Safety. ESCC can occur in both austenitic and duplex stainless steel equipment most often beneath thermal insulation. The insulation provides a medium to hold and move the water with its chlorides to the metal’s surface.
And believe it or not, early experiences of ESCC under insulation occurred under “wicking insulation”, which is the type of insulation we normally use to PREVENT CUI. But tests have shown that if this wicking insulation contained leachable chlorides, water permeating the insulation could extract chlorides and transport them to the stainless steel surface, which would cause ESCC. ESCC failures have also been reported under non-wicking insulation. In cases of non-wicking insulation, the water is
under the insulation, the chlorides dissolved in the water are from an external source or the atmosphere, not the insulation material.
When external water and chlorides enter around an inhibited, wicking insulation material, ESCC can develop because of the lack of silicate available on the wetted stainless steel surface. Plant experience shows that the inhibitor is not always leached out of the insulation in sufficient quantities, nor is the inhibitor always in the right place to inhibit the concentrated external chlorides. Sometimes the inhibitor may be leached so thoroughly under severely wetted conditions that it may be transported
away from the surfaces needing inhibition.
Sources of Chlorides
Testing and plant experience have shown that the chlorides more frequently come from coastal atmospheres (e.g. salt water), nearby chloride-containing chemical process units, wash water and fire protection deluge systems, and process spillage. Chloride concentration does not have to be high in the water, as the hot metal surface will concentrate the chlorides by evaporating to a level sufficient to cause ESCC.
We break these sources of chlorides down into two categories:
- insulating materials, including insulation, mastics, sealants, adhesives, and cement; and
- external sources
According to The National Association of Corrosion Engineers (NACE:
- Failures have occurred after ONLY A FEW YEARS OF OPERATION when the ESCC is associated with insulating materials containing high levels of leachable chlorides.
- Failures tend to occur AFTER FIVE YEARS OR MORE of service when the ESCC is caused by introducing chlorides from external sources, such as rain, coastal fog, deicing salt, wash water, fire, and deluge system testing, and process leaks or spills.
External sources account for most of the chloride-induced failures.
Sodium chloride (NaCl), or more commonly known as table salt, is the most prevalent chloride salt found in documented CUI events. When found in sufficient quantities, it can cause ESCC of the austenitic and duplex stainless steels. Other sources of chloride ions known to be aggressive include chlorine (Cl2), hydrogen (H2), chloride gas, hydrochloric acid (HCL), hydrolyzed organic chlorides, and thermally decomposed (at temperatures as low as 115°F) polyvinyl chloride (PVC). Likewise, acidic conditions in combination with chloride are more aggressive than neutral or basic conditions.
So yes, Stainless Steel piping and vessels can be impacted by CUI under the right situations AND locations. NEVER should we assume that we are “insulated” from a CUI issue on our process equipment merely because we have chosen SS over CS. I have even seen this occur on process equipment and the source of the chlorides was from a neighboring unit which was NOT a PSM/RMP unit!!! So be aware… SS is susceptible to CUI!