Effetto della diffusione dell'idrogeno sui fenomeni di Environmental Assisted Cracking di acciai per pipeline in condizioni di protezione catodica

Pipeline steels can show Environmental Induced Cracking phenomena under slow straining with Hydrogen Embrittlement mechanism under cathodic protection [1]. Hydrogen evolution can take place due to cathodic protection normally used in order to protect the pipeline against general corrosion. The steel is polarized at cathodic potentials in the range -0.8 to -1.1 V vs SCE, but very negative values could be reached in overprotected areas close to the impressed current anodes. The hydrogen ions reduction reaction takes place on the metal surface, creating adsorbed hydrogen. The adsorbed hydrogen diffuses into the metal owing its solubility in the metal lattice. In this paper hydrogen permeation tests were carried out according to the electrochemical methods proposed by Devanathan-Stachurski [8] on different pipeline steels (Tab. 1 and Fig. from 1 to 4). The hydrogen diffusivity depends on microstructure of the steel (Tab. 2). Also the tensile properties depend from the microstructure of the steel, so it is possible to observe a correlation between the hydrogen diffusion coefficient and the yield strength of the different steels (Fig. 5). Because hydrogen mainly diffuses through pre-existent paths as ferritic grain boundary [13], the diffusivity increases for the control rolled steel with respect to hot rolled steel owing to their fine ferritic grain. In the tempered martensite steel, the effect of very fine structures is contrasted by very fine and dispersed precipitates that can act as traps subtracting hydrogen to diffusion. Owing to yield strength depends on ferritic grain dimension and formation of hardening precipitates, a correlation between yield strength and diffusivity can be expected. However, figure 5 also shows important differences in behavior between steel with similar microstructures. The hot rolled steels by recent production have much lower diffusivities than the X60 steel produced in '60s, that has rather high content of carbon and sulfur and coarse microstructure with very pronounced pearlite band. In order to explain such a difference, less efficient hydrogen trapping could be supposed. The hydrogen diffuses in the metal lattice and concentrates in plastic deformation areas like the crack tip. The EAC phenomena can take place after the hydrogen concentration reaches a critical value. The hydrogen diffusion coefficients were correlated to the EAC parameters obtained in the SSR tests and in the corrosion fatigue tests. In the SSR tests the environmental effects are related to the decreasing of the reduction of area (Fig. 6-7 and Tab. 3). The ratio between the reduction of area in environment and in air, called normalised reduction of area, is a quantitative index of hydrogen embrittlement: the HE phenomena are more pronounced as this values is lower than one. The fatigue curves crack growth in air are in agreement with the Paris' law, (da/dN)F = CxΔK". (Tab. 4). In synthetic seawater under cathodic protection, the crack growth rate becomes much higher than the rate measured in air, at intermediate values of stress intensity factor range and low loading frequencies (Fig. 8). The crack growth morphology changes owing to the environmental effects as shown in Fig. 9. The effect is more pronounced as the cathodic polarization increases from -900 to -1050 mV vs SCE. At high stress intensity factor range, the curves in air and in synthetic seawater approach together. Mechanical factors prevail under crack instability conditions, as stress intensity factor approaches material fracture toughness. The crack growth rate becomes higher than the correspondent value in air above a threshold (ΔKeth) that can be related with a critical value of the maximum stress intensity factor. According to the Model of Wei and Landes [10], such