Corrosion resistance of duplex stainless Steel UNS S32101 and super austenitic stainless Steel UNS NO8031 (alloy 31) was investigated in crevice conditions in a corrosive environment (saline brine, T = 60 °C, C02-flow rate 3-5 L/h, normal pressure) using electrochemical and metallographic techniques. Rectangular rubber was used to form a metal/non-metal-crevice configuration.
Potentiodynamic measurements have been carried out in order to determine critical pitting and repassivation potentials. Potentiostatic measurements at different Potentials have been performed and the dependence of pit depth on the potential applied in crevice conditions was investigated. It was shown that the repassivation potential of duplex stainless Steel S32101 coincides with its corrosion potential. Strang pitting corrosion occurred after potentiodynamic measurements with pit depths up to 100 pm. Otherwise, stainless Steel UNS NO8031 shows about 0.5 V difference between the free corrosion potential and the repassivation potential and did not show any signs of pitting corrosion after polarization measurements. Slight rouging effects have been observed. By potentiostatic measurements on duplex stainless Steel S32101 potentials slightly more positive than the corrosion Potentials causing strong pitting corrosion, and pit depths up to 650 pm were measured. Super austenitic stainless Steel UNS NO8031 did not exhibit any signs of pitting corrosion even by applying potentials much more positive than the corrosion potential.
The reduction of the chromate contents in cements affects the electrochemical reactions of galvanized reinforcing steels in fresh concrete. By means of electrochemical measurements and bar pull-out tests the effect of chromates on the formation of protective coatings at the zinc and the bond behaviour have been investigated using chromate-containing and chromate-reduced cements. The results have shown that the bond zone was affected as a result of chromate reduction for cement types CEM I and CEM II. The disturbance to the bond zone should be considered as long-term in relation to the pore structure which is formed. The effects on the bond are largely compensated for as the concrete ages. The results of the related bond strength with displacement paths of 0.1 and 0.5?mm, respectively, are still within the range of expected figures for non-galvanized reinforcing steel despite the structural disturbance. Under practical aspects it is not necessary that the existing structural disturbance with respect to bond strength is taken into consideration.
Corrosion of steel reinforcement in concrete exposed to chloride containing environments is a serious problem in civil engineering practice. Electrochemical methods, e.g., potential mapping, provide information whether the steel reinforcement is still passive or depassivation has been initiated. By applying such techniques no information on the type of corrosion, its extent and distribution of corrosion products is available. Particular the corrosion progress is a significant problem. Especially in the case of macrocell corrosion in reinforced concrete structures, the development at the anode cannot be separated into corrosion damage resulting from macrocell corrosion or self-corrosion. Until now also in laboratory tests it is impossible to collect such information without destroying specimens after electrochemical testing was performed. To overcome this problem it was tried to study the steel surface within the mortar specimens by X-ray tomography (CT). Within the scope of these investigations it could be shown, that X-ray tomography is suitable to make corrosion pits and their development visible which are embedded in a mortar with a cover thickness of about 35 mm. In this publication the time-dependent corrosion damage of reinforced steel is documented by X-ray tomography.