Potential-time behavior, steel

Figure 25 Current versus time behavior for Type 302 stainless steel in 1,000 ppm NaCl at (a) a potential between its repassivation and breakdown potentials, and (b) at a potential below its repassivation potential. Note the existence of an incubation time before stable localized corrosion occurs in (a). The small, short-lived current spikes during the first 400 s are due to the formation and repassivation of metastable pits, which can also be observed in (b), although they are of a smaller magnitude.

Cast iron is initially anodic to low-alloy steels and not far different in potential from mild steel. As cast iron corrodes, however, especially if graphitic corrosion takes place, exposed graphite on the surface shifts the potential in the noble direction. After some time, therefore, depending on the environment, cast iron may achieve a potential cathodic to both low-alloy steels and mild steel. This behavior is important in designing valves, for example. The trim of valve seats must maintain dimensional accuracy and be free of pits consequently, the trim must always be chosen cathodic to the valve body making up the major internal area of the valve. For this reason, valve bodies of steel are often preferred to cast iron for aqueous media of high electrical conductivity. 

A prepassivated platinum electrode and an electrode of the metal of interest have been used to follow the development of a biofilm to determine its effects on the corrosion behavior of structural materials. The time dependence of the open circuit potential of several stainless steels... 

Fig. 2. Time-potential behavior of 18-8 stainless steel in tap water, and iron in 0.01 N K2Cr207, according to Burns (IS), showing a rapid change followed by a slow change.

From the data in Table 3.9, it appears that potential may on occasion be reversed even in distilled water heated to 85°C. The behavior in dilute potassium chloride is tabulated in Table 3.10. The chloride concentration is equal to that present in the tap water used in the investigation. The zinc did not become cathodic to steel at any time in this series. The presence of chloride seems to limit the occurrence of potential reversal. Mixtures of chloride with bicarbonate were also investigated. It is apparent from the data in Table 3.11 that reversals in potential can occur even in very dilute sodium bicarbonate solution when chloride is not also present. In mixtures of chloride and bicarbonates, the tendency to reversal depends on the relative concentrations of the two constituents. Bianchini et al. (I%8) also studied reversal in soft waters and various diluted aqueous solutions. Von Fraunhofer and Lu-binski (1974) investigated tap water and salt solutions, notably bicarbonate/ chloride. Ledion and Talbot (1969) found polarity reversal at 60°C in oxy-... 

Pitting of stainless steel (304 SS) was investigated by means of a SVET in combination with potential monitoring by Isaacs (1989). The in situ measurement of localized currents made it possible to follow the initiation, propagation, and repassivation of pits with a time resolution of a few minutes. Since the initiation of a pit leads to a decrease in the open circuit potential (OCP), while repassivation of a pit leads to an increase in the OCP, the potential transients proved to be a suitable complimentary method to monitor the activity of the sample. The behavior of stainless steel (in ferric chloride solution) was compared with that of iron (in a dilute chloride and sulfate solution). In the case of iron, it was observed that once started pits did not repassivate and finally spread due to a depassivation of the area next to the pit. 

Damborenea (1995) also studied the behavior of coated steels in aqueous media (NaCl 0.6 M and HCl 2 M) through voltametry experiments. It is observed that the values for the corrosion potential measured as a function of time range between 220 and 140 mV in the case of coated steels, and between -200 and -50 mV on uncoated steel. However, the kinetic measurements using complex impedance and polarization resistance showed that the resistance to corrosion diminished notably over time, with the R.p values reaching the same levels as those of naked steel after 24 h (Fig. 19-11). This downturn revealed the presence of defects or pores in the coating, which permitted the movement of ions and, consequently, the contact of the environment with the metal substrate. 

The corrosion potential of panel B also appeared to be unstable. Several times the open-circuit potential drifted to -3000 mV versus SCE. Every time this happened it was polarized at a normal potential for steel in the electrolyte. This unstable behavior is due to the very high resistance of the coating, which in this case exceeds the input impedance of equipment used. 

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