Tribocorrosion of high-strength low-alloy steels and coatings used in offshore applications

The use of High-Strength Low-Alloyed steels (HSLA) in offshore applications has risen considerably during the last decade. The increasing demand for lighter steels with higher strength has led to the development of steel grades with ultimate strengths up to 1100 MPa.

However, the high strength of these steels is achieved in detriment of other properties, and they are very susceptible to corrosion in marine environments, which comprise one of the most aggressive atmospheres. The likelihood of these steels of suffering substantial degradation with corrosion-related phenomena is high and involves significant costs of maintenance and replacement of damaged structures.

On the other hand, materials in offshore structures are also subjected to mechanical stresses such as wear. When wear and corrosion take place simultaneously, the process is known as tribocorrosion. There is a synergism between wear and corrosion, since the material loss when the two processes take place simultaneously can be considerably larger than when they occur alone. This leads to material losses higher than those collected in the design codes, which can have significant implications on the long-term integrity of components and structures. Therefore, a closer understanding of the tribocorrosion and the synergistic effect that it involves in HSLA steels is necessary to reestablish the wear-corrosion allowances in the design phase, in order to better predict the durability of high-performance materials in such adverse working conditions.

In this context, the first part of the present thesis deals with the assessment of tribocorrosion of HSLA steels used in chains of offshore mooring lines, which have been reported to be profoundly affected by both wear and corrosion during their service life. Up to now, tribocorrosion of passive materials has been widely reported, e.g., stainless steels, aluminum alloys, and titanium alloys. However, little investigations have been carried out on the response of active materials, such as HSLA steels of low corrosion resistance in the marine environment. In this regard, the present study aims to address the tribocorrosion of these materials on the basis of the procedures employed in passive materials, adapting and redesigning the protocols available in the standards and literature.

Additionally, the second part of the thesis is focused on the evaluation of protective coatings employed for the protection of steel components and structures from corrosion. To date, the selection of the coatings has been made mainly considering their effectiveness against corrosion, neglecting their performance against wear. For instance, organic coatings with high corrosion resistance due to their polymeric nature have been the most employed solution. However, their wear resistance is rather low, and any damage generated on the coating can considerably decrease their efficiency. In this framework, tribocorrosion studies have been performed for different coatings currently employed in the protection of offshore mooring lines, to verify their suitability against wear-corrosion requirements. Finally, a widely employed coating system has been functionalized through different surface modification techniques (Plasma Electrolytic Oxidation and organic coating functionalization), to enhance its performance against wear and corrosion and provide it with additional abilities such as superhydrophobicity and a biocide ability against certain bacteria.

All in all, the outcomes of this work show the importance of the synergism between wear and corrosion on the degradation of the HSLA steels, evincing the necessity of tribocorrosion related studies to increase the knowledge on this synergism and reestablish the allowances included in the design codes.