99 Diseases of Pressure Equipment: External Corrosion

For purposes of this article, external (atmospheric) corrosion is what afflicts process equipment and structural members that are not insulated and exposed to moisture associated with atmospheric conditions, ie rain, condensation from humidity, marine spray, cooling tower mists, industrial pollutants, etc. A US federal study indicates that the cost of corrosion in our industry in on the order of $30 billion/year (1), and I expect that the largest share of that cost is simple external corrosion.

As you might expect, corrosion rates range from less than 1 mpy in dry climates up to 20 mpy in tropical marine environments. Industrial environments (especially those in rainy humid areas) can cause external corrosion in the 5- 10 mpy range. The good news about external corrosion is that we don’t have to contend with insulation to inspect equipment. Visual inspection is key to finding external corrosion, and it’s usually fairly obvious because of the “ugly” corrosion products that accompany the metal loss. And if the equipment painting program is being kept reasonably up-to-date, external corrosion becomes even easier to spot. The bad news is that not all external surfaces of piping or vessels is readily exposed to visual inspection and external corrosion inspection can become a lower priority when compared to many of the 101 other essential elements of pressure equipment integrity management. For that reason, all three API Codes for inspection of vessels, piping and tanks (API 510/570/653) require external inspection planning at specific intervals or at intervals determined by a valid risk based inspection plan.

However, these requirements do not include all the structural steel that is exposed to external corrosion. Just last year, an operator at a mid-west refinery fell to his death when a safety guard rail broke. The reason for the breakage was allegedly due to external corrosion. This reinforces the notion that external structural inspections must also be planned and executed in addition to external pressure equipment inspections, though not necessarily by the same group of people.

Does your RBI program include the potential for external corrosion on structural members as well as process equipment?

Reference

1. Corrosion Costs and Preventative Strategies in the United States, U.S. Department of Transportation, Federal Highway Administration, FHWA-RD-01-156 & 7, March 2002