MechChem Africa September-October 2021

A corrosion investigation for a biokinetics pool This article outlines a Sassda case study involving pitting corrosion of a stainless steel support railing system following an upgrade to 316L material, which was expected to be an improvement on the original 304 system. Sassda’s acting Executive Director Michel Basson reports. S assda was recently cal led upon to comment on visible corrosion and pit marking on the stainless steel railing system, installed in a had been used. Figure 2 shows the damage to the material surface caused by pitting corrosion. At the lower right, a very rough surface finish is visible, while on the right, the mounting to the pool structure reveals the onset of crevice corrosion. Also to be noted are the unsealed crevices, which lack post-weld treatment, and the tube surface finish, which shows an incorrect grain direction.

tion, the opposite was found to be the case.

Material surface finish Such applications would normally make use of very highly polished surfaces. Sassda would ordinarily not advise using any sur- face finish less than #600 grid in this sce- nario and, judging from the visual evidence, the finish on the new railing components is not up to requirement and was found to be as rough as #180 grid. It was also noted that the finish was in the circumferential grain direction. When the components are installed horizontally, the grain direction points vertically and water that could possibly contain con- taminants can drain off. If a component is installed vertically, the grain direction becomes horizontal and the small ridges impede the washing off to remove any con- taminants or chloride deposits. The visual evidence also pointed to the fact that the welding was not properly smoothed down as expected for this type of application. The equipment should have had at least the same shiny surface smoothness as the hand railing and similar structures we find at airports and public places. Although the choice of grade 316 stainless steel was correct, the surface finish was not to the required standard. This could potentially limit the performance of the stainless steel and evidence points to this. Even with occasional spikes in chloride and chlorine levels in the pool water, it would not be expected that 316 would deteriorate at this rate. The risk of pitting is addressed by the fact that the water circulating in the pool will prevent stagnant conditions. There are, therefore, adequate oxygen levels in the water to maintain the protective chrome- oxide layer on the material surface. During the investigation no other chemical or forms of pollution could be identified as contribut- ing factors. Exposure to salt or harmful chemicals

biokinetics pool and necessary for safely supporting patients being treated in the water. Within six months of the pool being refurbished, the first visible signs of corro- sion and pitting became apparent on the newly installed railings. Although the pool is filled with a saltwa- ter solution, the chloride and chlorine levels were within the recommended corrosion protection range of the grade 316L material used. The pool temperature is maintained between 32 and 34°C and the pool is in an enclosed environment, with a high roof enabling humidity levels to be kept low. The grade of stainless steel used has been confirmed by spectrometer testing to be the specified austenitic grade, 316L. As seen from Figure 1, the railing com- ponents are submerged in the saltwater solution and never cleaned. However, the components are exposed to water being constantly circulated and therefore not oxy- gen deficient or stagnant. What is interest- ing about this investigation is the fact that the grade 316L railing systemwas installed to replace a grade 304 system which had been in use for four years before the first signs of discoloration and possible corrosion became evident. The replaced equipment was still available and spectrometer tests confirmed that grade 304 stainless steel

Failure of components is not usually caused by one incident or for one reason. It is common for failure to be attributed to a series of smaller, less critical factors and therefore important to investigate the following: Material grade As mentioned, the previous installation had been manufactured in grade 304 stainless steel and the new installation in grade 316. The owners of the pool reported good lifespan and performance from the origi- nal 304 stainless steel, while the new 316 grade showed signs of pitting as early as six months from start of use. Since grade 316 contains molybdenum it is especially protected in high chloride environments, so the 316 should have performed better in this appl icat ion. Technically, the original grade 304 should not have been suitable for the application since it lacks the specific protection against pitting corrosion offered by the molybde- num content in grade 316. In this installa-

Figure 1: A view of the biokinetics pool and its railing components, which are submerged in the saltwater solution and should not require cleaning.

34 ¦ MechChem Africa • September-October 2021