Friday, September 5, 2008

Roy Bash Comments

Solid film back pipeline coatings such as coal tar have the property to keep the pipeline perfectly dry. This is the reason that steel never corrodes under these coatings. Dry steel at normal operating temperatures never corrodes. These coatings are supplemented with cathodic protection for protecting the bare steel at places where the coating is not intact. The steel under the intact coating does not receive any cathodic protection current and it does not need any to prevent it from corroding.

Hydroscopic pipeline coatings such as FBE allow water molecules to pass through them, by the process of osmosis, to the pipe surface. This water is pure and pure water is non-corrosive to steel; hence no corrosion, cathodic protection or no cathodic protection.

From basic theory, the hydrogen ion consists of a single hydrogen atom minus its single valence electron embedded in a single water molecule. It is designated in electrochemistry text books as (H+.1H2O) or (H3O+).

When cathodic protection is applied, the FBE coating allows hydrogen ions to pass through it, by the process of electro-endo-osmosis, to the pipe surface. At the pipe surface the ion accepts one electron from the pipe and forms one atom of hydrogen which leaves one molecule of water at the pipe surface. Again, this water is pure and non-corrosive to steel, hence again no corrosion.

The water underneath a hydroscopic coating resulting through the process of osmosis is always pure and non-corrosive to steel. This is the reason that no significant corrosion is ever found under these coatings even in the water with hydrogen bubbles that are often observed under these coatings on cathodically protected pipelines when they are excavated and inspected. Just beware of the overly stringent -0.850v, CSE instant off cp criterion. History has proven the adequacy of the much less stringent -0.850v, CSE CP criterion measured with the CP applied (IR drop ignored) on steel pipelines with FBE coatings, and it reduces considerably the risk of forming the water bubbles compared to the overly stringent -0.850v, CSE instant off CP criterion which has the potential to completely disbond FBE coatings on buried or submerged pipelines over a period of time.

Respectfully submitted,
L.A.(Roy) Bash, P.E.

Ed Ondak Comments


I have not had the time to digest the document in its entirety. A quick glance, looked pretty good to me. One area that concerns me is 3.2. We should not reference regulatory requirements as a basis for the need for external corrosion. This document will be referenced world wide and some areas do not have regulatory requirements. No one, whether it be a company or an individual, should rely on and do a corrosion control program because a regulatory body says so. That is what brought about the regulations that we are faced with today.

A good corrosion program should be based on the science as we know it today, prudent operation by a company or an individual, considering public safety and the value of the assets that are being protected.

Remember, if we don't protect our assets, we may not have any assets to protect.

Edward Ondak, P.E.