Meccanismi di erosione dell’acciaio a contatto con calcestruzzo fresco

The paper deals with corrosion erosion of steel in contact with flowing fresh concrete. Metal devices in contact with fresh cementitious mixtures are subjected to severe damage due to erosion phenomena induced by flow. Thickness of components is seriously reduced and premature substitution must be considered to avoid perforation. This paper summarizes the results of erosion corrosion tests performed in fresh mortar on carbon steel and AISI316 stainless steel. The fresh mortar was manufactured by using chemical admixtures in order to retain constant workability during long time tests. SEM analysis was also carried out to evaluate damage morphology. Damage mechanisms and corrosion behavior are discussed as a function of the rotatinonal speed of electrode. No appreciable changes in the corrosion current density and the corrosion potential were noticed at speeds ranging from 0 to 10 rpm. At 100 rpm, the potential becomes more negative, as compared to values obtained without any electrode rotation, achieving very negative values at high rotational speeds. High angle impacts produce cracks and chips on the damaged surface. At low rotational speed metal surface shows a low number of straight ploughing craters caused by low-angle impacts of big particles. An increasing of the rotational speed above 100 rpm produces a damage on the whole electrode surface. The increase of the rotational speed of electrodes modifies both mechanical action of particles and mass transport phenomena of oxygen to the metal surface. The mechanical action due to the impacts of the particles determines an almost continuous scratching of metal surface and a constant exposure of new active sites. Simultaneously, after the breakdown, the protective film tends to rebuild according to its repassivation kinetics. The high content of solids in the cementitious mortar produces a great number of impacts that reduce the time available for repassivation. Thus, an incomplete state of passivity is stabilized that produces a mean value of passivity current density higher than values measured in the condition of the electrode at rest. At potentials between -200 mV and -800 mV vs. TiME, the cathodic polarization curves of the steels did not reach to current limiting conditions. The limiting current density was found at more negative potential values. The corrosion rate variations are substantially connected to the modification of anodic process, i.e. the passive current density. Moreover, the phenomena of mass transport do not appear to influence the corrosion rate in the range of rotational speed considered in the experimental work. For the fresh mortar considered in the experimental work, with 2 mm maximum size of aggregate, the critical rotational speeds for erosion were found in the range of 10 to 100 rpm, depending on material and geometry of rotating electrodes. The corrosion rate increases by one order of magnitude at about 300 rpm rotational speed.