Corrosion Engineering,Science and Technology: The International Journal of Corrosion Processes and Corrosion Control

The pitting corrosion resistance of DIN W. Nr. 1·4460 stainless steel (SS) with high amounts of nitrogen (0·87%) was evaluated to be used for medical implants. The SS pitting corrosion resistance was tested in a minimum essential medium at 37°C by electrochemical impedance spectroscopy and potentiodynamic polarisation curves and in a 0·1 mol L⁻¹ NaCl solution at 25°C, by scanning electrochemical microscopy. This last technique measures the concentration of chemical species released by corrosion processes. The potential of an ultramicroelectrode was set to amperometrically detect the Fe²⁺ ions released at the anodic areas and also the depletion of oxygen due to the cathodic reactions in the vicinity of the cathodic areas. The AISI 316L stainless steel was also tested for comparison reasons. The results showed that the DIN W. Nr. 1·4460 with 0·87% nitrogen presents higher pitting corrosion resistance than the AISI 316L SS, being a potential candidate for biomaterial applications.

The sol–gel method has been used to deposit oxide thin films. In the present study, silica was coated on 316L stainless steel substrate through sol–gel method via dipping process. Silica sol was made in a mixture of methyltriethoxysilane (MTES), tetraethyl orthosilicate (TEOS), pure water, absolute ethanol as solvent, nitric acid, and hydrochloric acid as catalysts. The corrosion resistance of samples was investigated in simulated body fluid by a potentiodynamic test and uniformity of the coating was evaluated by scanning electron microscope. We have assessed the effective parameters, such as wetting agent, number of layers, catalyst and different roughness of sublayers on corrosion properties of samples. The silica coating resulted in improvement of corrosion resistance of 316L stainless steel in body environment, and can be used as a proper coating on orthopaedic implants.

Calcareous deposits were formed on steel under cathodic protection conditions in artificial sea water at various potentials from −0·900 to −1·400 V(SCE). The deposition calcareous layers were characterised by electrochemical impedance spectroscopies, scanning electron microscopy observations and X-ray diffraction analyses. At 20°C, the deposits were composed of calcite CaCO₃ when formed at various potentials in solution 1, of brucite Mg(OH)₂ and aragonite CaCO₃ when formed at potentials from −0·900 to −1·200 V(SCE) and only of brucite when formed at potentials E⩽−1·300 V(SCE) in solution 2. Magnesium seems to influence the corrosion behaviour of freely corroding steel by causing calcium carbonate to precipitate as aragonite. Aragonite is more effective in covering the surface than calcite and is therefore more functional in preventing oxygen from reaching the steel surface, thereby lowering the corrosion rate.

Cathodic protection (CP) monitoring in the presence of simultaneous AC and DC interference could lead to erroneous measurements, since IR drop contribution due to both DC and AC could heavily affect potential readings. Therefore, to know the true potential (or true polarisation level), the ohmic drop contribution has to be eliminated. In literature, there is lack of agreement about the proper procedure to measure the true potential in the presence of AC. Laboratory tests on carbon steel specimens in soil simulating conditions were carried out focusing on potential measurement problem in the presence of AC through standard potential measurement procedure and the use of a potential probe. Results suggest the need of a proper methodology for potential measurement to determine reliably CP conditions.

Corrosion behaviour of copper and steel under heat trap conditions in Cuban tropical climate is reported. Temperature and humidity reach higher values than those reported for traditional outdoor and indoor conditions. Annual calculated time of wetness is in the range corresponding to outdoor or ventilated sheds. This behaviour is not reported for other indoor conditions. Sulphur compounds deposition rate is higher than chloride deposition rate at all corrosion stations. Main corrosion products formed on steel and copper are goethite and brochantite respectively. No significant differences in the statistical influence of exposure time and time of wetness on atmospheric corrosion process of copper and steel under heat trap conditions are determined.

A Ni based SY 625 alloy was oxidised at 900, 1000 and 1100°C under dry and wet conditions. Water vapour has little effect on the oxidation rate and scale composition. At 900 and 1000°C, the outer scale is composed of Cr₂O₃, and a continuous NbNi₄–Ni₃Mo subscale is found at the oxide/alloy interface. At 1100°C, the scale is composed of an outer chromia scale and an internal CrNbO₄ subscale. Nevertheless, the oxide scale morphology differs between dry and wet conditions. Under dry conditions, the oxide scale appears to be compact, and chromia pegs are observed at the internal interface. The oxide scales formed under wet conditions show that porosities spread inside the scale, and the chromia grain size is smaller. At 1100°C, some scale spallation is observed under dry and wet conditions probably due to the molybdenum oxidation, leading to MoO₃ evaporation and void accumulation at the internal interface.

Galvanised iron is popular in many applications, particular as a roofing material. However, just like other materials, especially metallic ones, it is prone to degradation by corrosion. In this particular study, the degradation of galvanised roof sheets was investigated at a coastal, urban and rural site in Tanzania, Africa. Samples were exposed to various outdoor environments over a period of 3 years. In addition, some accelerated laboratory investigations were conducted in different simulated air pollution environments in an artificial corrosion chamber constructed for this purpose to supplement the outdoor exposure tests. It was found that the combination of the tropical climate and increasing air pollution due to industrial development in the capital Dar-es-Salaam resulted in substantial atmospheric corrosion of the roof sheets, which eventually leads to failure and the necessity for replacement. The rural site had the lowest degree of atmospheric corrosion as expected. A combination of different corrosion products was identified as a result of the atmospheric corrosion by Raman and EDX analyses. The information gained from this investigation could be utilised to construct more durable structures requiring less frequent replacement and maintenance in future.

This paper investigated the effects of eight dissolved species concentrations on corrosion of carbon steel in water. Short term electrochemical experimental measurements indicated that the corrosion mechanisms of carbon steel in brackish water are uniform and pitting corrosion. The corrosivity of water is difficult to be defined using single parameter such as the corrosion rate. In this case, Mahalanobis–Taguchi method, as a discriminant analysis approach, is applicable for water corrosivity assessment. Mahalanobis–Taguchi method enables to predict whether a given water sample is acceptably corrosive or not. The preliminary investigation has indicated that Mahalanobis–Taguchi method can serve as a tool for water corrosity assessment.

This paper aims at the experimental investigation of time dependent non-linear relationship between critical carbonation depth and corrosion rate of steel in carbonated concrete by laboratory controlled experimentation under severe environmental condition. In this research, three-dimensional experimental observations are taken consecutively involving carbonation depth, half-cell potential and elapsed time as well as the gravimetric corrosion mass loss. The experimental observations revealed an interesting non-linear relationship between the above said measurements due to the varying resistivity of carbonated concrete. It is also found that the carbonation induced corrosion does not start until the carbonation depth reaches a certain critical level from the steel rebar and the half-cell potential values become constant after carbonation reaches the critical depth and then start rising again after carbonation reaches the rebar level.

A hybrid silica/epoxy nanocomposite film containing boehmite nanoparticles has been developed in this work through the sol–gel method to protect AA2024 alloy from corrosion. The hybrid sols were prepared by copolymerisation of 3-glycidoxypropyltrimethoxysilane, tetraethylorthosilicate and aluminium isopropoxide. The films were prepared by dip coating technique. The morphology and the structure of the hybrid sol–gel films were studied by scanning electron microscopy and atomic force microscopy. The corrosion protection properties of the films were investigated by potentiodynamic scanning and electrochemical impedance spectroscopy. The results indicate that the presence of the boehmite nanoparticles in hybrid structure of the silica/epoxy films, highly improved the corrosion protection performance of the coating system.

Static state immersion experiments, polarisation curve and electrochemical impedance spectroscopy tests have been applied to investigate the corrosion kinetics for both spark plasma sintered (SPS) and conventional sintered Nd–Fe–B magnets in NaCl electrolyte. Effect of microstructure modification on their chemical stability of the magnets was discussed. The electrochemical reactions of both magnets are controlled by the step of active substances adsorption process at the open circuit potential and the anodic potential, which turn to diffusion process at the cathodal potential. Although both magnets are susceptible to corrosion in saline electrolytes, SPS magnets are more corrosion resistant than conventional sintered magnets due to their special microstructure that is different from those of conventional sintered magnets. In SPS magnets, the grain size of the Nd₂Fe₁₄B main phase is fine and uniform, only a few Nd rich phase form along the grain boundaries of Nd₂Fe₁₄B phase, while most of them agglomerate into triple junctions as small particles. Such microstructure effectively restrains the aggressive intergranular corrosion along Nd rich phases. As a result, the SPS magnet possesses excellent corrosion resistance in NaCl electrolyte.