Corrosion of Metals by Acid River Water. I : Theoretical Consideration of Corrosion of Iron and Steel

The mechanism of corrosion of iron and steels by acid river water was theoretically studied. The corrosion was found to be either a catnodic or a mixed control. In the stagnant acid water, the rate-determining step of the corrosion reaction was the diffusion of dissolved oxygen (diffusion control) at higher pH than 3.6, whereas it was the discharge of hydrogen ions (activation control) at lower pH than 3.6. The corrosion rate at the activation control was calculated from the heat of adtivation of the rate-determining step, the pH and the natural electrode potential. The corrosion rate in the mixed control was calculated from the heat of activation, the pH, the standard single electrode potential, the diffusion constant of metal ions and the thickness of diffusion layer. The heat of activation was calculated from the cathodic current density and the overvoltage. In the flowing acid water, the rate-determining step was the diffusion of hydrogen ions at higher pH than 4.1, whereas it was the diffusion of dissolved oxygen at lower pH than 4.1. The corrosion rate was proportional to seven-eighth power of the flow velocity. Stainless steels might be passivated in the high speed flow. It was shown that the "moto-electrode effect" was important on the corrosion in the acid river water

Surface Bond between Organic Corrosion Inhibitors and Metal Surfaces

The delocalization energy in the surface bond formation is estimated by the method proposed by Grimley, using a linear crystal model and the "tight-binding" approximation. The change of the energy with the electronegativity of a foreign atom and a surface metal atom is discussed. The change of polarization of electrolytic iron and pure nickel by the addition of an inhibitor in dilute mineral acids is in general agreement with the theoretical deduction attained on the assumption that the surface bond is formed by over-lapping of a lone pair electron orbital of an inhibitor molecule and a vacant orbital of a surface metal atom

The Mechanism of Stress-Corrosion Cracking in the Brass-Ammonia System

The effect of corrosive environments, compositions of alloying elements and degree of pre-strain on the stress-corrosion cracking of the brass-ammonia system, has been investigated and the mechanism of intergranular cracking and of the transition from intergranular to transgranular cracking have been discussed. Intergranular cracking arises from the formation of thick and large cuprous oxide grains over slip steps and the resulting locking of the movement of dislocations. The transition from intergranular to transgranular cracking depends on the mechanical properties of the surface film produced in a corrosive environment. The mechanism of the stress-corrosion cracking has been explained on the basis of the film theory

Study on the Corrosion Resistivity of Al-Mg Alloys by means of a Potentiostat

The electrochemical behavior of Al-Mg alloys were studied by means of a potentiostat. Since oxide films can be easily formed on Al-Mg alloys, both components of which are active, the reproducibility of results obtained by the potentiostat was not so good. From the polarization curves in acidic, neutral and alkaline aqueous solutions, however, the effects of the magnesium content could be seen. The results of the potentiostatic measurements were in good agreement with those obtained by ordinary corrosion tests. Anodic polarization of an Al-1% Mg alloy in aqueous solutions of hydrochloric acid or sodium chloride sometimes showed abnormal behavior. The surface of these alloys were examined by the electron diffraction method. According to the electron diffraction patterns, formation of MgAl_2O_4 films could be observed only on the surface of an Al-1% Mg alloy. This film would be more stable to chlorine ions than the surface films of pure aluminium and high magnesium content alloys. The effect of aging on Al-Mg alloys was recently studied by Jacquet and his co-worker only in the anodic region. In the present research, this effect on Al-5% Mg alloy was measured both in the anodic and the cathodic regions. In the anodic region, the degree of etching was dependent on aging time, and cathodic polarization was also dependent oh aging time ; namely, the longer the aging time, the more sensitive the alloys were to corrosion. From these results, it was concluded that the potentiostatic measurements could be applied to the study of aging of light alloys

Dezincification of Alpha Brass

Dezincification of α-brass, which is one of the most prominent cases of selective corrosion, has been investigated by the electron diffraction method. To explain the process of the dezincification, two hypotheses are considered, one of which can very likely explain our experimental results. In the early stage of the dezincification, sharp diffraction rings from copper begin to appear together with those from the α-brass which is the initial specimen. With the progress of the dezincification, the intensities of the copper diffraction rings increase gradually and those of α-brass decrease. The dezincified α-brass layer composed of pure copper has a textural property similar to, but not strictly the same as, that of the initial specimen. It is concluded that the process of the dezincification is the result of dissolution of α-brass into corrosive solutions followed by re-deposition of copper on the specimen surface