ABSTRACT

External pitting corrosion is a serious concern for offshore stainless steel vessels, tanks and piping. Protective coating is an economic option to mitigate the pitting corrosion of offshore stainless steels. However, in the literature, there is a lack of publications on the selection of proper coating systems on pitted stainless steels to minimize pitting growth and prevent new pitting formation.

The objective of this study was to 1) develop a test protocol to test coating performance on pitted stainless steel 2) compare the performance of protective coating systems on various surface preparations, 3) determine a proper coating application procedure to cover the pits, to improve the integrity of offshore pitted stainless steels.

The following lab tests were performed to determine the performance of various coating systems: 1) adhesion test, 2) water vapor permeation, 3) underfilm corrosion, and 4) holiday-free film formation. As a result of this work, protective coating systems and the application method for the pitted offshore stainless steel facilities were identified. The most critical issues for the selection of coating systems for offshore pitted stainless steels are to have a good adhesion durability and low water permeation. The primer must be applied with brush to fill the pits and the rest of coats can be applied by spray.

INTRODUCTION

Offshore stainless steel (SS) vessels and pipes in Gulf of Mexico (GoM) have suffered pitting corrosion. Due to variation of pit shapes and morphologies, it is very difficult to assess and predict the remaining service life of the pitted stainless steel pipe and equipment. It is very desirable to select a suitable protective coating system to minimize the pitting growth and to prevent new pitting formation. A qualified coating system leads to significant cost, time and resources reduction, as compared with the replacement of the existing offshore pitted stainless steel equipment.

The purpose of this work is therefore to:

(1) Develop a test protocol to qualify protective coating system(s) to minimize pitting growth and prevent new pitting initiation,

(2) Evaluate various surface preparation methods on the coating performance on stainless steels,

(3) Identify a reliable coating application method to cover pits with a holiday free film on pitted stainless steels.

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