Steel electrochemical mechanisms

Paints used for protecting the bottoms of ships encounter conditions not met by structural steelwork. The corrosion of steel immersed in sea-water with an ample supply of dissolved oxygen proceeds by an electrochemical mechanism whereby excess hydroxyl ions are formed at the cathodic areas. Consequently, paints for use on steel immersed in sea-water (pH 8-0-8-2) must resist alkaline conditions, i.e. media such as linseed oil which are readily saponified must not be used. In addition, the paint films should have a high electrical resistance to impede the flow of corrosion currents between the metal and the water. Paints used on structural steelwork ashore do not meet these requirements. It should be particularly noted that the well-known structural steel priming paint, i.e. red lead in linseed oil, is not suitable for use on ships bottoms. Conventional protective paints are based on phenolic media, pitches and bitumens, but in recent years high performance paints based on the newer types of non-saponifiable resins such as epoxies. 

Aluminum powder, in particular, is frequently employed at relatively high concentrations in high-temperature epoxy adhesive formulations. The filler provides improvement in both tensile strength and heat resistance, and it increases the thermal conductivity of the adhesive. Aluminum powder fillers also reduce undercut corrosion and, hence, improve adhesion and durability of epoxy adhesive between bare steel substrates. It is believed that this is accomplished by the aluminum filler providing a sacrificial electrochemical mechanism.27... 

Gao, M., Chen, S., and Wei, R. P., Electrochemical and Microstructural Considerations of Fatigue Crack Growth in Austenitic Stainless Steels, 36th Mechanical Working and Steel... 

Figure 7.1 Electrochemical mechanism of corrosion of steel in concrete [5]...

In a third procedure, an adhesion promoter of a 3-(ethyl-phosphonic-acid)thiop-hene was applied to the metal surface followed by an electrochemical film preparation [65]. The following procedure was used in this case Mild steel was mechanically polished, then the specimen were treated for 60 minutes in a solution of 3-(ethyl-phosphonic-acid)thiophene. Afterwards, a layer of poly(3-methylthiophene) was formed by electropolymerization in an electrolyte consisting of 0.1 mol 1 3-methylthiophene, 0.1 moll tetrabutyla-mmonium-hexafluorophosphate (N(Bu)4 PFg) in dichloromethane (CH2CI2). 

When steel or iron is exposed to an atmospheric environment, a thin layer of magnetite, Fe304, is formed, covered by a layer of FeOOH. Atmospheric oxygen then penetrates though the almost water-free, porous outer layer of FeOOH and oxidizes the magnetite to hydrated ferric oxide, Fe203, or FeOOH. The presence of Fe " in the electrolyte initiates the precipitation of various corrosion products. The electrochemical mechanism of atmospheric corrosion of iron suggested by Evans is briefly summarized in this chapter [8]. 

Sathiyanarayanan, S., (eyaram, R., Muthukrishnan, S., and Venkatachari, G. (2009) Corrosion protection mechanism of polyaniline blended organic coating on steel./. Electrochem. Soc., 156, C127-C134. 

Figure 4.4.55 presents the real vs. imaginary components of the electrochemical-mechanical impedance response measured for an FIY80 steel specimen of geometry shown in Figure 4.4.53 immersed in 3.5 wt % NaCl. Due to the form of Eq. (177), these plots are somewhat more complex than a conventional Nyquist plot. Nevertheless, these data are amenable to standard methods of electrical... 

Figure 4.4.55. Complex plane plot of the electrochemical-mechanical impedance for the propagation of a crack through HY80 steel in 3.5% NaCl solution at 25°C under sinusoidal loading conditions.

Aluminum powder is often included to modify the thermal conductivity of epoxies. It also reduces undercut corrosion and hence improved adhesion and durability of epoxy adhesives between bare steel substrates, perhaps by a sacrificial electrochemical mechanism. 

Corrosion also occurs as a result of the conjoint action of physical processes and chemical or electrochemical reactions (1 3). The specific manifestation of corrosion is deterrnined by the physical processes involved. Environmentally induced cracking (EIC) is the failure of a metal in a corrosive environment and under a mechanical stress. The observed cracking and subsequent failure would not occur from either the mechanical stress or the corrosive environment alone. Specific chemical agents cause particular metals to undergo EIC, and mechanical failure occurs below the normal strength (5aeld stress) of the metal. Examples are the failure of brasses in ammonia environments and stainless steels in chloride or caustic environments. 

Wilde, B. E. and Kim, C. D., The R61e of Hydrogen in the Mechanism of Stress-corrosion Cracking of Austenitic Stainless Steel in Hot Chloride Media , Corrosion, 28, 350 (1972) Lin, F. and Hochman, R. F., Electrochemical Study of Stress-corrosion Cracking of Ti 8-1-1 Alloy and NaCl Solutions , Corrosion, 28, 182 (1972)... 

Szklarska-Smialowska, Z., Electron Microprobe Study of the Effect of Sulphide Inclusions on the Nucleation of Corrosion Pits in Stainless Steels , Br. Corros. J., S, 159 (1970) Weinstein, M. and Speirs, K., Mechanisms of Chloride-activated Pitting Corrosion of Martensitic Stainless Steels , J. Electrochem. Soc., 117, 256 (1970)... 

Herbsleb, G. and Schwenk, W., Electrochemical Study of the Pitting Corrosion of Corrosion-resistant Steels , Werkst. Korros., 24, 763 (1973) C.A., 88. 21970h Poatsch, W., Optical Studies of the Mechanism of Pitting Corrosion , Ber. Bunsenges Phys. Chem., 77, 895 (1973) C.A., 80, 90201v... 

In recent years the mechanism of crevice has been mathematically modelled and a more thorough understanding of the corrosion processes has been evolved . From such mathematical modelling it is feasible to predict critical crevice dimensions to avoid crevice corrosion determined with relatively simple electrochemical measurements on any particular stainless steel. 

Entry from the aqueous phase The mechanism of electrochemical production of hydrogen on steel in aqueous solution has received much attention. It is accepted that the reaction occurs in two main stages. The hrst of these is the initial charge transfer step to produce an adsorbed hydrogen atom. In acid solution this involves the reduction of a hydrogen ion ... 

There are several classes of test for hydrogen embrittlement, according to the application. Three general types of mechanical test can be identified, together with chemical and electrochemical tests intended to determine the hydrogen content of steels or the rate of entry of hydrogen from an environment. 

The behavior of materials, particularly steel, in cavitating fluids results in an erosion mechanism, including mechanical erosion and electrochemical corrosion. The straightforward way to fight cavitation is to use hardened materials, chromium, chrome-nickel compounds, or elastomeric plastics. Other cures are to reduce the vapor pressure with additives, reduce the turbulence, change the liquid s temperature, or add air to act as a cushion for the collapsing bubbles. 

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