Sunday, February 21, 2010

Comments from Mauro Chaves Barreto

Richard,



Thanks for email below. It makes us thinking about this issue which is always an educational process. I usually agree on what you say, but this time I found something that I think you misunderstood. I do not think this draft asks anyone to measure corrosion rate. It just says that both criteria have empirical evidences to reduces corrosion rate down to 25µm/y. On the other hand, evidences that you reduced corrosion rate down to this level, does not mean that you comply to the statement. I say this because all corrosion rate measurement methods have flaws and coupons are not pipelines. Maybe a good comment to the negative is a better clarification about this issue.



Also, I think there is a big misunderstanding behind this long discussion in general. I think very low corrosion rate does not mean CP and immunity provided by CP is not always necessary for corrosion control. I can give you two practical examples about what I said:

- A pipeline operator wanted to assess pipeline integrity after 25 years operation. Their idea was to run a smart pig, but operational conditions did not allow them to do it at that time. They performed CIS and found a spread of 8 km without any potential shift. It happened because pipeline was in a rock trench that shielded CP current. They made several dig-ups and found no evidence of corrosion even with a deteriorated coating. My recommendation was to run smart pig and do nothing in case of no external corrosion is found.

- A consultant from our company inspected a very big heat exchanger in a power plant and recommended galvanic CP as the mitigation method. He designed the system without any way to measure potentials, since stationary reference electrodes were very difficult to install. Client asked how he can be sure that system was working properly and he just said that it can be done visually just stopping equipment after one year operation. It was done some weeks ago and system was just perfect.



So, my question is: why do we need to prove that any structure, including pipelines, have potentials that would reduce corrosion rates in almost all situations? If we can prove that corrosion rate is acceptable and under control, do we need to prove that we also have -0,85V ON or OFF or whatever? In the past, this discussion for pipelines would not make much sense, since there was no other way to “prove” low corrosion rate besides measuring pipe to soil potentials, but today we have smart pigs.



I will think better about how to incorporate these thoughts in my probable negative. I would appreciate your comments.





Mauro Chaves Barreto

IEC - Instalações e Engenharia de Corrosão Ltda.

No comments: