What Does Couper-Gorman Curves Mean?
Couper-Gorman curves are curves on a graph which represent the influence of temperature and hydrogen sulphide concentration on corrosion rates of carbon steel. They are used to predict corrosion rates in hydrogen-containing environments.
Couper-Gorman curves are generally believed to provide the most practical correlation among corrosion rates, temperature and concentration of hydrogen sulfide for various steels. These correlations are widely used for material selection.
Couper-Gorman curves available for:
- 5Cr-0.5Mo steel
- 12% Cr stainless steel
- 18Cr-8Ni austenitic stainless steel
Corrosionpedia Explains Couper-Gorman Curves
Couper-Gorman curves provide basic information for material selection for refinery hydrogen units through predicting vulnerability to corrosion. The curves were created based on a survey conducted by National Association of Corrosion Engineers (NACE) Committee T-8 on Refining Industry.
For low-alloy steels two sets of curves apply, depending on whether the environment is gas oil or naphtha. No corrosion occurs at low hydrogen sulphide concentration and temperatures exceeding 600ºF (315ºC). Total process pressure of 1 - 18 MPa was found to have no meaningful influence on the corrosion rates.
These curves differ from other previously published research through reflecting influence of temperature on corrosion rates throughout a whole range of hydrogen sulphide concentration. Total pressure was not significant.
From Couper-Gorman curves, it may be seen that the corrosion rates of carbon steel and 5Cr-0.5Mo steel are similar. Addition of 9% of chromium improves the corrosion resistance only by a small degree, and a distinct improvement is provided by 18% chromium in austenitic steel.
Couper-Gorman curves provide a reasonably good prediction tool for high-temperature systems containing hydrogen sulfide and hydrogen. Unfortunately, these prediction curves have proven unreliable in certain circumstances, particularly in the hot distillation sections of hydrocracker units.