Abstract

Rust creepage or undercutting from a discontinuity in a coating film, such as scribe or holiday, is an important mode of coating degradation on steel substrates. The mechanism of rust creepage is attributed to cathodic disbondment. This paper describes an experimental study of the influence of steel substrate topography, measured with a digital replica tape reader, on rust creepage. For the two coating systems that were studied, it was found that rust creepage was strongly inversely correlated with substrate peak density.

Definitions

The following terms used in this paper are defined as follows:

Disbondment: The loss of adhesion between a coating and the substrate.

Rust Creepage (Undercutting): The spread of corrosion or disbondment from a scribe or holiday in a coating film.

Introduction

The spread of disbondment or corrosion from a scribe or holiday in a coating film, for which the terms rust creepage or undercutting are used in this paper is an important mechanism of coating degradation. The mechanism of rust creepage has been well studied by several authors who concur that the mechanism is driven by electrochemical reactions15. The reactions occurring at the discontinuity in the coating (scribe or holiday) involve an anodic reaction in which iron is dissolved. Adjacent to the anodic region, under the coating, a cathodic reaction occurs in which oxygen is reduced to hydroxyl ions. The negatively charged hydroxyl ions formed underneath the coating attract cations to the cathodic areas. It is thought that the combined destructive effect of an alkaline environment, peroxides, and free radicals weakens the bond between the coating and the substrate. This causes a ‘cathodic disbondment’ front to advance from the discontinuity into the intact coating regions which undercut the coating. As well as undercutting, blistering may also occur near the discontinuity. The term ‘cathodic disbondment’ is also used to describe the similarly electrochemically driven disbondment that occurs at a discontinuity (‘holiday’) in a coating under applied cathodic protection.

Keywords:
flowline corrosion, production chemistry, pipeline corrosion, upstream oil & gas, rust creepage, corrosion inhibition, subsurface corrosion, h2s management, oilfield chemistry, retrieval system
Subjects:
Production Chemistry, Metallurgy and Biology, Pipelines, Flowlines and Risers, Materials and corrosion, Well Integrity, Subsurface corrosion (tubing, casing, completion equipment, conductor), Corrosion inhibition and management (including H2S and CO2)
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