Analysis of metallic coatings based in zinc-aluminium-magnesium alloys, in terms of performance and long-term corrosion. Case studyelectrical cable trays selection in project design

Resumen

In recent years, numerous types of surface corrosion coatings, based especially on zinc-aluminium-magnesium alloys (known as "ZM" alloys), have been developed as alternatives to traditional zinc-based coatings (known as "Z"), to improve its technical characteristics and reduce its cost. The manufacturers of these new treatments claim greater resistance to corrosion, based on accelerated corrosion tests and field tests, the latter lasting only a few years. The main objective of this thesis is the structuring and analysis of all the existing information in the current state of the art, and in particular, the study of the existing field tests to corroborate their resistance to corrosion in different types of environments and based on this, propose a mathematical model that facilitates its long-term calculation. A review of the state of the art of metal coatings based on ZM alloys is presented, which covers their evolution over time, the different qualities and designations existing in the market, their structure and composition, international standards that regulate them and a detailed research on field tests in different locations around the world, having found tests of a maximum duration of 6 years. From the analysis of these field tests, a methodology is proposed to verify the performance and evolution of the corrosion-time function in the different exposure environments, categorized through the international standard ISO 9223 (ISO, 2012), which calls them "corrosivity classes", and which range is from C1 (very low) to CX (extreme). This analysis has classified all the test results by material, corrosivity class and evolution over time. In this way, each corrosivity class has been investigated in depth, through statistical analysis, with special emphasis on annual corrosion, measured as mass loss (µm / year), the corrosion-time function and its adjustment to a certain behaviour. The Z coatings have also been analysed to be able to compare both alternatives and corroborate the main hypothesis, whose main assumption is the greater resistance to corrosion of ZM alloys compared to Z coatings. This analysis has been the entry point to establish a mathematical model that determines the long-term corrosion performance, to provide project engineering professionals, with a tool to estimate the corrosion resistance and optimize the cost of an installation when different types of materials are used. The summary of all this analysis has been reflected in the Results and discussion section. The referred methodology has been applied to a case study to show how to select the quality of the coating and its optimal thickness, as well as a cost calculation, in order to guarantee the requirements of a specific project, in terms of resistance to corrosion and cost. The final conclusions show that there are some advantages of ZM alloys over Z coatings, mainly with regard to corrosion resistance, having found relationships that can double and triple their performance, in the periods for which there are data available. In the same way, some disadvantages have been found, which must be investigated further in future research works, to give continuity to this thesis. For example, the limitation of these coatings to achieve large thicknesses, the limited duration of field tests, the performance of specific parts of the components (cuts, embossments, bends, welds ...), etc.