Steel, rest potential

Fig. 2-23 S-N curves for plain carbon steel in 0.05 M potassium hydrogen benzoate (pH 4) at 30°C at various potentials U U = rest potential).

Stainless steel-soil Rest potential — About -0.4 to +0.4 Soil... 

Greater deviations which are occasionally observed between two reference electrodes in a medium are mostly due to stray electric fields or colloid chemical dielectric polarization effects of solid constituents of the medium (e.g., sand [3]) (see Section 3.3.1). Major changes in composition (e.g., in soils) do not lead to noticeable differences of diffusion potentials with reference electrodes in concentrated salt solutions. On the other hand, with simple metal electrodes which are sometimes used as probes for potential controlled rectifiers, certain changes are to be expected through the medium. In these cases the concern is not with reference electrodes, in principle, but metals that have a rest potential which is as constant as possible in the medium concerned. This is usually more constant the more active the metal is, which is the case, for example, for zinc but not stainless steel. 

This criterion is derived from the fact that the free corrosion potential in soil is generally I/cu Cuso4 -0-55 V. Ohmic voltage drop and protective surface films are not taken into consideration. According to the information in Chapter 4, a maximum corrosion rate for uniform corrosion in soil of 0.1 mm a can be assumed. This corresponds to a current density of 0.1 A m l In Fig. 2-9, the corrosion current density for steel without surface film changes by a factor of 10 with a reduction in potential of about 70 mV. To reduce it to 1 jum a (0.14 V would be necessary. The same level would be available for an ohmic voltage drop. With surfaces covered with films, corrosion at the rest potential and the potential dependence of corrosion in comparison with act contrary to each other so that qualitatively the situation remains the same. More relevant is... 

As in the case of corrosion at the insulating connection due to different potentials caused by cathodic protection of the pipeline, there is a danger if the insulating connection is fitted between two sections of a pipeline with different materials, e.g., mild and stainless steel. The difference between the external pipe/soil potential is changed by cell currents so that the difference between the internal pipe/ medium potential has the same value, i.e., both potential differences become equal. If the latter is lower than the former for the case of free corrosion, the part of the pipe with the material that has the more positive rest potential in the soil is polarized anodically on the inner surface. The danger increases with external cathodic protection in the part of the pipeline made of mild steel. 

In some service stations, stainless steel or aluminum materials are used for all the filters, pipes and fittings to maintain the purity of the fuel. The rest potentials of these materials are different from that of plain carbon steel (see Table 2-4). 

The danger of corrosion on buried installations in industrial plants is increased by various soils and by cell formation with cathodes of steel in concrete. The rest potentials of these foreign cathodes are between = -0.2 and -0.5 V [4-6]. 

Double Loop EPR Test Details of this procedure are given in Japanese Industrial Standard JIS G 0580 (1986). The sample is ground to a 1(X) grit finish then placed in the test solution for about 2 min to establish the rest potential (about —400 mV (S.C.E.) for AISl Types 304 and 304L stainless steel). 

The CPE appears to arise solely from roughening of the surface by the corrosion process. This was verified with IS experiments on iron and several steels in 15% HC1 at 25°C. The electrodes were polished with alumina and maintained at 150 mV cathodic of the rest potential. Complex plane plots of the impedance responses were nearperfect semi-circles centered on the V axis. Analyses via EQIVCT using the Rq+P/R circuit, gave rise to n-values of the CPE in excess of 0.93 in all cases and remained constant throughout the tests. 

Table 1.6 Rest potential of sulphide, sulfarsenide and arsenide minerals of steel media at near neutral pH...

With their rest potential much higher than that of the steel medium listed in Table 1.6, sulphide minerals act as a cathode while the steel medium acts as an anode. During grinding, minerals and grinding media (steel) come in repeated contact with each other, and galvanic current flows between the two surfaces of the sulphide minerals may be altered. 

Twelve different Bonderite 40 coated steel (B40) panels were examined to provide a statistically valid value for the open circuit potential. Their average rest potential was -0.578 V (vs. SCE) with an average deviation of 20 mv. After recording the open circuit potential, polarization curves were obtained (Figure 1). 

The effect was more pronounced at the starting potential than at the finish potential. Leidheiser suggested that the best performance is obtained when the cathode/anode surface area ratio is the same as the uncoated metal. Inadequate performance is obtained when the cathode/anode area ratio becomes larger. Qur work agrees with Leidheiser s hypothesis. The B210/GBL coatings have rest potentials less noble than the B40 coated steel panels and perform best in the salt fog environment. 

Figure 8. Weight loss and rest potential of mild steel in 10% HtSO as a function of the concentration of p-naphthoquino-line and o-tolylthiourea (3)...

The current density of various materials was determined as a function of the potential difference between the anodic and cathodic branches of the current potential curves in 0.9% NaCl with a stable redox system Fe (CN)6" 7Fe (CN) [1]. The saline solution containing this redox system had a resting potential closely resembling that of a tissue culture fluid which has a redox potential of 400 mV. Ti and its alloys Ta and Nb exhibit a better resistance than the stainless steel AISI 316L and a wrought CoNiCr alloy. The same ranking can be observed during the measurement of the polarization resistance of the different materials [1]. Breakdown potential measurements of various implant materials in... 

The danger of corrosion on buried installations in industrial plants is increased by various soils and by cell formation with cathodes of steel in concrete. The rest potentials of these foreign cathodes are between t/cu-cuS04 = and -0.5 V [4-6]. Factors that affect cell formation are the type of cement, the water/cement ratio and the aeration of the concrete [6]. Figure 12-1 shows schematically the cell action and the variation of the pipe/soil potential where there is contact with a steel-concrete structure. The cell current density is determined by the large area of the cathode [see Fig. 2-6 and Eq. (2-44)]. In industrial installations the area of steel surface in concrete is usually greater than 10 m ... 

To ensure long durability, electrical grounding equipment in power stations is constructed of corrosion-resistant materials that have a very positive rest potential (e.g., copper with = -0.1 to -0.2 V). These grounds, like steel in con-... 

Berthome, G. and Baroux, B., "Crevice Corrosion of Stainless Steel Electrochemieal Noise Analysis at Rest Potential," New Trends in Electrochemical Impedance (EIS) and Electrochemical Noise Analysis, PV2000-24, F. Mansfeld, E. Huet, and O. R. Mattos, Eds., The Electrochemical Society, Pennington, NJ, 2001. 

Figure 28 Rest potential/time curves for hot-dip galvanised steel in Ewersbach tap water as a function of the water temperature [57]...

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