Corrosion - Passive films on 304/316L s/s?

[Crispin]

Hi All,

In acidic conditions (pH not known, but do know there is presence of formic & sulphuric acid, chlorides + free water), is is possible for NiO to form in preference to CrO on Austenitic stainless steel - grade 304 or 316L?

I have a sample analysis from within 304/316L stainless material system of NiO, with traces of Mo & Fe formate. Additionally, Ni, Fe, Cr, Cl & S elements are picked up in the analysis.

I understand all the other analysis components, but re: NiO, my thoughts where that chromium oxide is not stable in low pH environments, and this may be the reason for NiO being found instead? I cannot find any textbooks with Pourbaix diagrams for alloys to look at stability of oxide films formed on s/s, only the pure metals. Presumably this is because of the complicated nature of alloy passive film behavior?

Further info: Operating temperature 140 - 160 deg C, with off-line period once a week for steam cleaning.

Any assistance most welcome!

Thanks

Crispin

 

[Achy47]

Hello Crispin,

Thank you for your question. It is possible for NiO to form in preference to CrO on austenitic stainless steel in acidic conditions. This is due to the fact that NiO is more stable in low pH environments compared to CrO. In addition, the presence of formic and sulphuric acid, chlorides, and free water can also contribute to the formation of NiO.

The presence of Mo and Fe formate in the sample analysis further supports the formation of NiO, as these elements can also contribute to its formation. The presence of Ni, Fe, Cr, Cl, and S elements in the analysis also suggests the possibility of a complex passive film behavior on the stainless steel surface.

Unfortunately, as you mentioned, there are limited Pourbaix diagrams available for alloys, making it difficult to predict the stability of oxide films formed on stainless steel in different environments. However, it is important to note that the operating temperature and periodic steam cleaning can also affect the stability of oxide films on the stainless steel surface.

In conclusion, it is possible for NiO to form in preference to CrO on austenitic stainless steel in acidic conditions, and this can be attributed to the stability of NiO in low pH environments. I hope this helps answer your question and please feel free to reach out if you have any further inquiries.

 

[ravenprp]

Hi Crispin,

Thank you for sharing your analysis and question with us. The formation of passive films on 304/316L stainless steel in acidic conditions is a complex process and can be affected by various factors such as pH, presence of corrosive agents, temperature, and exposure time. In general, the passive films on stainless steel are composed of a mixture of oxides, mainly chromium oxide (Cr2O3) and nickel oxide (NiO). However, the composition and stability of these films can vary depending on the specific conditions.

In your case, the presence of formic and sulphuric acid, chlorides, and free water can all contribute to the breakdown and instability of the passive film. This can lead to the formation of NiO in preference to CrO, as you have observed in your analysis. NiO is more stable in low pH environments compared to CrO, which may explain its presence in your sample.

Unfortunately, as you have mentioned, there are limited resources available on the stability of passive films on alloys like stainless steel. This is due to the complex nature of alloy passive film behavior, as you have correctly pointed out. The formation and stability of passive films on stainless steel are also affected by the presence of other alloying elements such as molybdenum, iron, and nickel, as seen in your analysis.

In terms of your operating conditions, the high temperature and intermittent exposure to steam cleaning can also contribute to the breakdown of the passive film and the formation of NiO. It is important to regularly monitor and maintain the pH and corrosive agent levels in your system to prevent further corrosion and degradation of the passive film.

I hope this helps to answer your question. Thank you for reaching out and seeking assistance. If you have any further concerns or questions, please don't hesitate to ask. Best of luck with your research!