Evaluation of Aluminum-Sprayed Coatings for Corrosion Protection of Offshore Structures
Authors: Salama Mamdouh M. , Thomason William H. 
Published: December 2007

Selection of an optimal corrosion protection method for offshore structural high-strength steel was necessary to avoid potential fatigue and hydrogen embrittlement problems. A study to evaluate the corrosion protection provided by sacrificial anodes and flame-sprayed aluminum coatings was provided by sacrificial anodes and flame-sprayed aluminum coatings was undertaken. Reliability, availability, and influence on fracture and fatigue properties were the main factors considered in the study. The results illustrated that a properly applied flame-sprayed aluminum (FSA) coating represents the most viable scheme for providing long-life corrosion protection and improved corrosion fatigue and cracking resistance. protection and improved corrosion fatigue and cracking resistance.

Introduction

The predicted gradual depletion of fossil fuels is confronting the oil industry with the challenge of drilling and producing oil and gas wells from offshore reservoirs in deep water and adverse environmental conditions. Fig. 1 shows the historical and projected water depth for exploration and production activities. To meet this challenge, several new structural concepts have been proposed for development of fields in deep water. All these concepts are classified as compliant structures. Compliant structures, such as tension leg platforms (TLP's), are characterized by their ability to move significantly in response to environmental loads. Conoco Inc. installed the first TLP (Fig. 2) in July 1984 in the North Sea (Hutton field). The use of high-strength steel in the TLP for fabricating several key structural systems, such as the mooring and riser systems, is necessitated by the desire to reduce the TLP displacement and minimize the need for complicated tensioning and handling systems. Both mooring and riser systems are subjected to more than 108 loading cycles during the service life of the platform. This makes corrosion and corrosion fatigue resistance important design parameters. Therefore, the selection of a system that achieves long-term corrosion protection and minimizes the influence of the environment (seawater) on protection and minimizes the influence of the environment (seawater) on fatigue resistance is essential to ensure the integrity of high-strength steel components. This paper discusses the results of a program to develop an optimal corrosion protection system for high-strength steel components submerged in seawater.

Corrosion Protection Options

Three types of corrosion protection were considered: cathodic protection, inert coatings with cathodic protection, and anodic coatings. Cathodic protection requires that the steel be polarized to some negative potential protection requires that the steel be polarized to some negative potential at which the dissolution of iron is suppressed. This is generally achieved with an impressed current system, sacrificial anodes of either zinc or aluminum, or a combination of these. This approach is the most common for the protection of offshore structures. Inert coatings include conventional epoxies, fusion-bonded epoxies, and heat-shrink sleeves. The coating provides a physical barrier as long as they are continuous over the provides a physical barrier as long as they are continuous over the components. Cathodic-protection backup is required to protect uncoated areas or areas where coating damage has occurred. Anodic coatings include metal-sprayed zinc and aluminum coatings. They remain effective even when the coating is ruptured locally. The concept of anodic coatings is similar to cathodic protection by sacrificial anodes.

Cathodic Protection

Corrosion protection using aluminum anodes represents a conventional approach and a well-proved method. However, this method suffers from two drawbacks when used for high-strength steel components. First, the currently available zinc and aluminum alloys can produce cathodic potential on the steel that approaches - 1050 mV with produce cathodic potential on the steel that approaches - 1050 mV with respect to saturated calomel electrodes (SCE). This cathodic level can result in hydrogen embrittlement of the high-strength steel. To assess the impact of this problem on the 3 1/2 Ni-Cr-Mo-V high-strength steel used for fabricating the mooring system for the Hutton TLP, slow strain rate, hydrogen permeation, and corrosion fatigue studies were performed at different potentials. The slow strain rate test is merely a tensile test at a very low crosshead speed. The results shown in Fig. 3 demonstrate the susceptibility of high-strength steel to hydrogen embrittlement at potentials below –800 mV (SCE). The results of seawater corrosion fatigue tests performed at 68F [20C] and at different potentials using 1/4-in. [6.4 mm] diameter specimens subjected to potentials using 1/4-in. [6.4 mm] diameter specimens subjected to tension-tension loading are presented in Fig. 4. The results show that the maximum fatigue strength appears to exist around the –900-mV (SCE) potential. The results in Figs. 3 and 4 show that the optimal cathodic potential. The results in Figs. 3 and 4 show that the optimal cathodic protection system would need to provide a cathodic potential slightly below protection system would need to provide a cathodic potential slightly below the –900-mV level to achieve the maximum fatigue life and minimize the effect of hydrogen embrittlement. This cannot be guaranteed with a conventional sacrificial anode system. The second drawback is related to providing a reliable electrical contact between the sacrificial anode and the high-strength steel components.

JPT

p. 1929