Weight reduction of vehicles (WRV) is a continuous challenge from the beginning of the vehicle history, however the purpose of WRV has been changing by social requirements. Recently automotive industry is facing with the global warming and the other environmental issues, so we are sharing a vast amount of R&D resources to improve the fuel economy. In this paper, the changes of the purpose of WRV and, the relation between fuel economy and WRV, are summarized. And also the current status of light metals usage and the future work for the related material issues, in particular the corrosion resistance, are briefly mentioned.
Recently, reduction of component weight for gas mileage improvement in automobile applications has become a subject of great importance for environmental protection. This paper describes some applications of corrosion protection technologies and corrosion evaluation technologies for automotive heat exchangers made from aluminum alloys. Corrosion environments discussed in this paper are de-icing salt splash on radiators and condensers, engine coolant water in radiators and heater cores, and condensate solutions on evaporators.
The recent trends on corrosion protection technology for improvement of strength on steels, especially in automotive application, are reviewed. The decrease of susceptibility to environmental cracking, such as, delayed fracture, and corrosion fatigue is the key point for the improvement of strength for quench and temper type high strength steels for high tensile bolts, suspension springs, door beams, etc. Delayed fracture of high tensile bolt steels is minimized by change of lubricant, improvement of Zn electroplating condition, materials improvement such as precipitation of hydrogen trapping site, etc. Corrosion improvement is also very effective for decrease of delayed fracture of door beam steel. Corrosion fatigue life improvement of high strength spring steel is achieved by improving resistance to pitting corrosion and hydrogen embrittlement. Its pitting corrosion is minimized by rust control, and the hydrogen embrittlement by the introduction of a hydrogen trapping sites in the matrix. Corrosion protection of steel sheet for automotive body, tire steel cord, etc. are also reviewed.
The corrosion behavior of carbon steel in 5.0M MDEA and DGA solutions was investigated by potentiodynamic polarization and impedance measurements under 4.5MPa carbon dioxide (CO2) at 100°C. The corrosion-product layers formed on the carbon steel in these conditions were examined using scanning electron microscopy (SEM) and X-ray diffraction (XRD). A tight corrosion-product layer, which consists of iron carbonate (FeCO3) crystal, was formed on the carbon steel in the 5.0M MDEA solution, but a loose corrosion-product layer, which consists of iron carbide (FeC3) and FeCO3, was formed in the 5.0M DGA solution. In the 5.0M MDEA solution, the corrosion rate of carbon steel was low (-1μAcm-2). The anodic and catholic reactions were inhibited by the tight corrosion-product layer. Particularly, the diffusion of the species responsible for the catholic reaction was strongly inhibited by the layer, and of course, that was the determining-step. In this case, the corrosion potential was about -0.6V vs. Ag/AgCl, and the main catholic reaction may be the reduction of carbonic acid (H2CO3). Contrarily, in the 5.0M DGA solution, DGA-carbamate formed a soluble complex with Fe(II), and thereby carbon steel corroded at a high corrosion rate (-2×103μAcm-2). The corrosion potential was about -0.8V vs. Ag/AgCl, and the main catholic reaction may be the reduction of DGA-ammonium ion.
Corrosivity of vapor-phase environments in indoor pool, water tank, and water purification plants was investigated. Sodium hypochlorite (NaClO) was used as a sterilizing agent in indoor pool, while chlorine gas was used in water tank and water purification plants. It was found that Cl- ion were concentrated in the dew formed in the indoor pool. H+ ions as well as Cl- ions were accumulated in the dew formed in the water tank and water purification plants. Thus, the corrosion conditions was varied with the type of sterilizing agents used. Through the investigation of water tank, the relationship between pH and Cl- ion concentration was given as follow; pH=-1.09-2.19log[Cl-] (mol/L). Corrosivity of vapor-phase environments in sterilizing water systems would be characterized by the existence of oxidizing chemical agents such as ClO- and HClO, the shift of corrosion potential of the thin water film, and the accumulation of H+ and/or Cl- ions in the dew.
Sewerage systems have recently come into wide use in Japan. The pipes and fittings in the systems are exposed not only to the corrosive environment but also to the impact of solid particles contained in the mud drain resulting in erosive wear on the surface. To cope with the trouble, it is intended to replace the traditionally often used iron and steel pipes with those of polymers which have excellent anti-corrosion properties. In this study, slurry erosion test were carried out using a Jet-in-slit apparatus on seven kinds of polyethylenes, three kinds of other polymers and two kinds of iron and steels taking it thoroughly into account that the erosion damage depends remarkably on the particles impact angle. As a result, all the polyethylenes showed an excellent anti-erosion properties as compared with other materials for whole range of particles impact angle. Further, a model was proposed to account the impact angle dependency of the erosion damage. Basing on the model it was made clear why the erosion rate of the polyethylenes were satisfactory correlated with a complex parameter in which the fracture energy and the elastic modulus were combined.
In order to clarify wire diameter dependency of corrosion rate of steel wires, which have been widely used in various fields, immersion tests and polarization tests in pH 5.9 and pH 2.5 NaCl solutions were conducted to high carbon steel wires with 0.1 to 1.0mm in diameter. It was found that the corrosion rate was largely dependent upon the diameter in both solutions, that is, it increased with the decrease of the diameter. Diffusion limiting current was observed in both solution, and it also had the same diameter dependency. Corrosion rate and diffusion limiting current in pH 2.5 solution were about one order larger than those in pH 5.9 solution. The analysis of diffusion limiting current and diffusion model for cylinder-shaped wire electrode based on Fick's law indicated that corrosion rate was controlled by diffusion of dissolved oxygen in pH 5.9 solution and that by diffusion of hydrogen ion in pH 2.5 solution, and that the increase of corrosion rate with the decrease of wire diameter was attributable to promotion of cathodic reduction reaction of dissolved oxygen in the neutral solution, and that of hydrogen ion in the weak acid solution, respectively.
When stainless steels samples are immersed in natural sea water, an ennoblement of the electrode potential (Esp) up to approximately 400mV vs. SCE can occur, due to the development of biofilm on the samples. The main organisms in the biofilm include bacteria and diatom. In this paper, it was investigated the relationship between the ennoblement of Esp and bacteria species in the biofilm, and the interaction between bacteria and diatom in the Esp ennoblement. To study effect of various bacteria species in the biofilm on the ennoblement of Esp, type 316 stainless steel samples were immersed in 0.4μm filtered natural sea water, where the filtering was expected to restrict bacteria as the only organisms in the water. Under this condition, ennoblement of Esp up to 400mV vs. SCE could not be attained, and the need of other organisms in biofilm, besides bacteria, to promote ennoblement was shown. The analysis of biofilm, formed in natural sea water, on sample with ennobled Esp, detected the presence of sulfate-reducing bacteria, acidic and neutral sulfur-oxidizing bacteria (A-SOB and N-SOB, respectively). It was also observed that the density of N-SOB was high all through the year. To study effect of SOB on ennoblement of Esp, type 304 stainless steel was immersed in solutions containing Thiobacillus thioparus (as N-SOB) and T. thiooxidans (as A-SOB). Remarkable Esp ennoblement was induced by N-SOB, but not by A-SOB. However, as the density of attached N-SOB on the ennobled sample was too much higher than the density to be attained under immersion in unfiltered natural sea water, the ennoblement of Esp was concluded not to occur by that single bacteria. When samples were transferred to diatom containing solution, after immersion in natural sea water for a few days, to promote attachment of bacteria on the samples, the Esp of the samples presented high values, similar to the values observed for samples immersed in natural sea water in summer. After immersion in diatom containing solution, the effect of the density of attached bacteria and diatom on Esp was investigated. For Esp ennoblement up to 200mV vs. SCE, the density of attached bacteria increased up to about 3.5×104pg/cm2, with increasing density of diatom. For Esp ennoblement higher than 200mV vs. SCE, it is suggested that the attachment of diatom on the sample plays a more important role.