ABSTRACT

Thermally sprayed aluminum (TSA) has been used to protect offshore structures for decades. Their effectiveness in mitigating corrosion of steel structures in seawater has been demonstrated over the years. However, very little work is reported on the corrosion performance of TSA when damaged. Furthermore, data on the performance of damaged TSA in deep sea is virtually non-existent. To address these knowledge gaps, Al was sprayed onto carbon steel and defect (holiday) amounting to 3% of the sample area was drilled to expose the underlying carbon steel substrate. The sample was exposed to synthetic seawater at 5°C in an autoclave at 50 MPa to simulate 5000 m of water pressure for 90 days. After testing, examination of the sample revealed the formation of calcareous deposit on the exposed steel surface with no visible steel corrosion product. Detailed microstructural characterization of the calcareous deposit confirmed the formation of protective Mg-and Ca-containing layers. Thus, one can conclude that TSA has the potential to protect carbon steel in deep sea environment even when damaged.

INTRODUCTION

Marine corrosion is a major concern for organizations operating in the offshore sector. To mitigate corrosion of offshore assets various mitigating methods are used. One such method is the use of thermal spray coatings such as those of aluminum and its alloys. It is well established that thermally sprayed aluminum (TSA) coatings provide long-term corrosion protection of offshore steel structures.1,2 The low self-corrosion rate of aluminum in seawater coupled with its ability to act as an anode with respect to steel makes it an ideal candidate for such applications.3-5 This anodic nature of aluminum offers sacrificial protection to carbon steel in seawater. The polarization results in increased interfacial pH which leads to the deposition of calcareous matter on any exposed steel.6 Such deposits have the favorable effect of reducing the cathode area (exposed steel) and hence decreasing the corrosion rate.7

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