Corrosion Damage to Vessels Have to Be Accurately Measured & Documented

I found myself as corrosion / integrity consultant of a low pressure vessel  built to API 620 standards.  The vessel was in the process of being recoated after ten years of service.  During removal of the coating internal and external corrosion damage was reported to exist on the vessel.   My company was asked to do a integrity accessment and recommend how the corrosion damaged vessel was to be repaired.

The corrosion damage reported on the internal surface didn't appear consistent with internal corrosion damage.  A NACE International Level 2 Coating Inspector with a ultrasonic thickness gauge had done the measurements.  In addition, the corrosion damage was not reported in a format that allowed any calculation of the remaining strength of the vessel wall.

An API inspector and myself entered the vessel to inspect the internal corrosion damage.  We found construction defects which didn't impact the integrity of the vessel being improperly called out as corrosion damage with inaccurate ultrasonic wall thickness measurements.  Basically we found no evidence of any internal corrosion pitting and very minor construction defects which didn't impact the integrity of the vessel wall.

The coating inspector didn't use a pit gauge but put his ultrasonic thickness gauge on rough surface manufacturing defects which caused inaccurate measurement of deep corrosion pitting.  Research on the vessel operating conditions indicated that internal corrosion was highly unlikely as the conditions inside the vessel didn't support corrosion.  Therefore, the fact that the API inspector and myself couldn't find any internal corrosion pitting was to be expected.  This type of vessel in this service has a history of zero internal corrosion issues.

Nine locations of external corrosion pitting caused by coating failure in a Pacific coast atmosphere had resulted in the need to do weld-fill repair which took about a week to complete.  These corrosion pits were small in size yet deep enough to require repair.

It is important that anyone doing ultrasonic wall thickness measurements on a API 620 or similar vessel know ASNT, API, and ASME methods to accurately measure and document corrosion pititng damage.  A coating inspector may be qualified to do coating inspection, but not qualified to document corrosion damage to a vessel.