Failure Investigation (Corrosion)

Introduction

Stainless steel elbows undergo a pickling and passivating process. Some of these elbows exhibited white spots on the surface following this process. The elbows have already been installed and cannot be removed to do the necessary testing to determine the cause of the irregularities. It was suspected that the elbows were left in the pickling solution for too long thus creating the imperfections on the surface.

1 (Custom)

Figure 1: Installed elbow

Investigation

One Eighty was tasked with recreating the spots on a new elbow in a controlled environment, thus enabling an investigation into the cause of the white deposit.  Further, the long term effect of this damage would have to be assessed in terms of the corrosion performance.  The elbows are AISI 304 stainless steel and the pickling solution used to clean and passivate the stainless steel is Dipsol-S.

What We Found

Results

EDS analysis on the clean surface and pitted areas found no difference in the chemical composition of the two regions.  The spectrographic analysis shows that the steel complies with AISI 304 stainless steel and no compositional defects exist in the stainless steel. The standard operating procedure for sample preparation of the pickling treatment is as follows:
  • Degrease
  • Dry
  • Submerge in the pickling solution for 2-12h depending on the acid strength and ambient conditions
  • Clean thoroughly
  Samples of an untreated elbow (Figure 2) were cut and given the standard pickling and passivation treatment. 2 (Custom)

Figure 2 Untreated Elbow Surface

Initially, two samples were submerged in the pickling solution, one for two hours and one for five hours. The surfaces from both samples appeared to be cleaner and shinier after the pickling, with no adverse affects present. No evidence of the white deposit and surface damage could be seen on either sample (Figure 3). 3 (Custom) 4 (Custom)   Figure 3 a) Front After 2h in Pickling Solution. b) Back After 2h in Pickling Solution. The entire elbow was then submerged for twenty-four hours in the pickling solution. This treatment is twelve hours longer than the recommended treatment for this particular part.   White deposits can be seen, and the surface is heavily pitted (Figure 4). These features are similar to those seen on the installed elbow. 5 (Custom) 6 (Custom)   Figure 4 a) and b) Surface after 24h in picking solution Sections of the untreated elbow and the sample treated for twenty-four hours were polished and examined under the light microscope (Figure 5). The untreated sample has a smooth surface finish compared to the jagged surface after twenty-four hours in the pickling bath. A clear effect of the long term exposure to the pickling solution is noticeable between the two surfaces. It is clear that the long term exposure to the pickling solution has damaged the surface of the stainless steel.  It can be assumed then that the passive layer that should have developed on the surface of the stainless steel during pickling and passivation has not properly occurred.  The jagged surface features together with the disrupted passive layer make it likely that these areas will not perform as expected and will be subject to corrosion and surface discolouration and damage. 7 (Custom) 8 (Custom) Figure 5 Light Microscope Images a) Original elbow, b) After 24h in pickling solution

Conclusion

  • The elbow section has an alloy composition that complies with the both the standard AISI 304 specification and the certificate of analysis.
  • The recommended pickling and passivation process causes the stainless steel to become shiny and bright, as is the intention of the process. No surface damage was found.
  • Exposure to a twenty-four hour pickling treatment results in damage similar to that observed on the elbows in service.  Therefore the surface damage observed is a result of over exposure of the order of twenty-four hours to the pickling and passivation process.
  • The damage resulting from over exposure to the pickling and passivation process will cause the elbow section to be compromised in terms of corrosion performance.

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