Direct Examination Results of High Strength versus Low Strength Steels The Dilemma Found during Investigative Digs

摘要

Due to metallurgical improvements in the steel industry, higher strength steels have made significant strides in: specified minimum yield strength (SMYS), wall thickness control, toughness, weight per foot, etc. However, one significant factor in these improvements was "no consideration for corrosion allowances". Corrosion allowances in steel have decreased with every incremental increase in the improvements of making of higher strength steels. Corrosion in carbon steel is the same whether it is a 35,000-yield pipe or an 80,000-yield pipe; or 0.375" wall thickness versus 0.215" wall thickness. Example: a 36.8 % wall loss on 35,000 yield pipe results in a 73.6 % loss on higher strength steel pipe. Lower strength steel direct assessments can result in recoat type repairs, while higher strength steels may result in composite repairs or replacement. Unfortunately, the US coating industry and pipeline operators have not addressed these changes to offset or compliment the improvements made in higher strength steel. To address the dilemma, either add more corrosion allowances to the steel, improve coatings or do both. Coatings are supposed to be the first line of defense against corrosion, while cathodic protection is secondary. Is this really occurring? The findings of direct examination digs appear to show otherwise.