Authors: Diego Pesenti Bucella
Source: Open MIND (article, 2019-01-01)
Field: Materials Science — Metals and Alloys

Cathodic protection and hydrogen embrittlement The main topic of the research activity carried out at the Department of Engineering and Applied Sciences of the University of Bergamo during this 3-year PhD Program was the study of hydrogen embrittlement in high strength low alloy steels under cathodic protection. The project of this PhD Thesis is entitled ?Cathodic Protection and Hydrogen Embrittlement?, and the entire research activity was financed by APCE Service Srl (Associazione per la Protezione dalle Corrosioni Elettrolitiche). The necessity of deepening this very wide and complex theme was driven by the purpose of a more efficient prevention and control of the corrosion mechanisms that can verify, in particular operating situations, in underground steel pipelines and transport systems, in which the combined use of a cathodic polarization and a protective coating must be necessarily involved. Cathodic protection (CP), along with the application of protective coatings, represents one of the main techniques for corrosion protection of submerged parts of metal structures exposed to the marine environment, buried structures, and equipment operating with natural and process waters. The extensive experience has made it one of the most reliable protection techniques, essential to guarantee full safety and long service lives in naval, offshore and underground structures, Oil&amp;Gas equipment, transport systems and pipelines, etc. It is usually applied to protect carbon and low alloy steels in neutral or slightly alkaline solutions, in order to reduce the overall loss of metal and to enhance the corrosion-fatigue limits. Protection is achieved by means of a cathodic current, flowing from the anode towards the structure to be protected, sufficient to lower the metal potential at any point of the surface below a protective limit, the so-called protection potential (Ep). To less noble potentials of Ep, the general corrosion rate reduces to less than 10 ?m/year, or it becomes nil if the polarization leads to immunity conditions. The effectiveness of this protection technique is indubitable. However, there may be particular conditions in which CP, especially if not correctly applied, can produce negative effects. The most important effect, analyzed in this PhD Thesis work, is that connected to Hydrogen Embrittlement (HE), a specific type of Environmental Assisted Cracking (EAC) connected to the absorption and consequent diffusion of atomic hydrogen through the metal matrix. The risk of such effect becomes evident mostly for excessive cathodic polarizations, when over-protection potentials (Eop) are reached, well below Ep, in which the polarization cathodic current density becomes high and the process of hydrogen development becomes relevant. Therefore, the aim of this PhD Thesis was to better understand the process of hydrogen diffusion in a commercial pipeline steel, because the presence of hydrogen in metallic materials is well known to be detrimental for the mechanical properties in certain conditions, as it causes significant decrease in ductility and/or fracture strength, unexpected failure, etc. Measurements of hydrogen permeation in metallic materials were carried out for nearly 40 years according to the electrochemical permeation technique proposed by Devanathan and Stachurski and used in the experimental tests, which probably provides the simplest and most flexible approach. However, even if many results were published since then and several methods were developed to evaluate hydrogen uptake and transport, the interpretation of the literature data and the correlation with the proposed models was not always satisfactory. The structure of this PhD Thesis is organized in a general section (from Chapter 1 to Chapter 6) and in an experimental section (from Chapter 7 to Chapter 10). As regards the general section, Chapter 1 illustrates the application of CP to buried pipelines in order to eliminate or reduce the corrosion rate of the coating defects exposed to the soil environment down to negligible values, along with the importance of the application of a protective coating on a steel pipeline, which must ensure the physical separation of the steel towards the environment. In fact, even the most efficient and high-performance coating present hidden or non-systematic defects, which the final qualification tests are often unable to detect. Consequently, the application of a cathodic polarization must be necessarily undertaken. However, in correspondence with very negative potentials, the steel is in over-protection conditions. Atomic hydrogen, developed from the cathodic reaction, diffuses through the steel and can lead to the formation and propagation of cracks; this risk increases with the increase in the mechanical strength of the steel. The most important national and international reference standards indicate precise limits for the application of CP to avoid the risk of embrittlement, especially in steels with a high Yield Strength (YS); the critical limit is generally indicated as the potential for the beginning of hydrogen development. Chapter 2 explains the hydrogen induced failure mechanisms related to EAC, subsequent to hydrogen evolution and permeation into the metal matrix, when a metallic material is not protected against an aggressive environment, represented by different kinds of soil in the case of underground pipelines. A quite exhaustive overview of the main theories proposed as a possible explanation for Hydrogen Assisted Cracking (HAC) mechanisms is then reported. In Chapter 3, after the explanation of the pure diffusion mechanism, in accordance with Fick?s first and second law, attention was paid to the effects of microstructure, temperature, sub-surface hydrogen concentration, applied mechanical stress, specimen thickness, and trapping sites. In Chapter 4, the main mathematical models for hydrogen diffusion are presented, both in the case of ideal diffusion (absence of trapping sites) and non-ideal diffusion (presence of trapping sites, i.e. sites that affect hydrogen diffusion and can represent preferential paths or traps, which are usually classified as reversible or irreversible in relation to the binding energy). Chapter 5 will provide an interesting examination of the mathematical models that try to explain the effect of elasto-plastic deformation on hydrogen permeability. In Chapter 6, the electrochemical permeation technique, proposed by Devanathan and Stachurski for the evaluation of hydrogen diffusivity through steel, and used in the further tests, will be analyzed. Chapter 7 describes the experimental permeation tests and procedures performed on one type of high strength low alloy carbon steel, catalogued as API 5L X65 grade steel, which is probably one of the mainly utilized in pipelines construction for the transportation of petroleum and natural gas. The experimental results deriving from the permeation tests, realized in accordance with the International ISO 17081:2014 standard in the absence of an applied load and in the presence of cyclic and incremental step loading conditions, are presented in Chapter 8. The discussion of the results achieved is reported in Chapter 9. For the processing of the experimental curves, firstly the implementation of the pure diffusion model (according to Fick?s second law) was proposed; however, this resulted not accurate enough to simulate the permeation process of hydrogen, due to the existence of trapping sites. Therefore, another processing method, proposed by Grabke and Riecke, allowed to calculate diffusion parameters in a more accurate way than the previous model, also in the presence of residual plastic deformation or loading conditions beyond the yield limit. In Chapter 10, the conclusions to this PhD Thesis work are drawn. From the analysis of the main international and national standards that regulate the application of CP, it was concluded that no criteria are standardized for the determination of the critical limit potential indicated to avoid the occurrence of HE phenomena. This limit should be experimentally determined by means of mechanical tests, but without any precise indication of the test methods to be adopted. As regards the permeation tests performed in the absence or in the presence of an applied load, the results allowed to better understand the variations in hydrogen transport mechanism into a X65 grade pipeline steel. In particular, with the application of cyclic loading conditions beyond the yield limit, it was observed: - Significant decrease in the apparent diffusivity, due to the enhancement of trapping phenomena - Sharp increase in the reversible trapping parameter, due to the contribution of the accumulation of new trapping sites in the plastic deformation field. The extent of the plastic deformation achieved in the tests is relatively low, thus the delaying effect related to the irreversible traps is small if compared to the multiplication of reversible traps - Significant increase in the total hydrogen concentration, as a result of enhanced hydrogen absorption and filling of an increasing number of trapping sites - Appreciable mitigation of the stress field generated by a tensile stress after the application of a compressive stress, with a consequent less marked decrease in Dapp - Temporary reduction in hydrogen flux, determined by a variation of the applied maximum stress, due to an instantaneous reduction of the mobile hydrogen concentration in the lattice, caused by an increase in the number of trapping sites following local plasticization phenomena even for stresses lower than the yield limit. Concerning the permeation tests performed in the presence of an applied incremental step load, for the X65 grade steel (sorbite) and heat treated (martensite) material, it was observed that: - Failure occurred at stress values very close to those in air, and in a region very far from the permeation area, with no crack propagation during the constant deformation phase Cathodic protection ; Hydrogen embrittlement ; Corrosion ; Galvanic anode ; Coating ; Anode ; Embrittlement Hydrogen embrittlement and corrosion behaviors in metals ; Structural Integrity and Reliability Analysis ; Corrosion Behavior and Inhibition