Metastable corrosion pits

The onset of pitting corrosion occurs suddenly If one performs electrochemical experiments with stainless steel, e. g. by applying a constant electrical potential to a sample immersed in dilute NaCl solution, the electrical current - which is an indicator for chemical activity (corrosion) on the metal surface - is low over a wide parameter range. But if critical parameters like temperature, potential, or electrolyte concentration exceed a certain critical value, the current rises abruptly and the metal surface is severely affected by pitting corrosion. The transition to high corrosion rates is preceded by the appearance of metastable corrosion pits. 

Metal dusting usually occurs in high carbon activity environments combined with a low oxygen partial pressure where carburisation and graphi-tisation occur. Usually pits develop which contain a mixture of carbon, carbides, oxide and metal (Fig. 7.52). Hochmann" proposed that dusting occurs as the result of metastable carbide formation in the high carbon activity gas mixture which subsequently breaks down into metal plus free carbon. The dependence of the corrosion resistance of these nickel alloys on the protective oxide him has been described accelerated or internal oxidation occurs only under conditions that either prevent the formation, or lead to the disruption, of this him. In many petrochemical applications the pO is too low to permit chromia formation (ethylene furnaces for example) so that additions of silicon" or aluminium are commonly made to alloys to improve carburisation resistance (Fig. 7.53). 

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.

The most common electrochemical test for localized corrosion susceptibility is cyclic potentiodynamic polarization. As was discussed briefly in the section on the electrochemical phenomenology of localized corrosion, this test involves polarizing the material from its open circuit potential (or slightly below) anodically until a predetermined current density (known as the vertex current density) is achieved, at which point the potential is scanned back until the current reverses polarity, as shown in Fig. 42. The curve is generally analyzed in terms of the breakdown (Ebi) and repassivation potentials (Elf). Very often, metastable pits are apparent by transient bursts of anodic current. The peaks in current shown in Fig. 42 for a potentiodynamic scan are due to the same processes as those shown in Fig. 25 for a potentiostatic hold. 

Figure 4 Metastable pitting rate (k) for 316 stainless steel in 1 M NaCl solution as a function of potential for untreated samples and samples passivated with 20% or 50% nitric acid for 1 hour. (From J. S. Noh, N. J. Laycock, W. Gao, D. B. Wells. Corrosion Sci. 42, 2069 (2000).)...

Chromates are particularly effective inhibitors, and there appear to be several components to inhibition. Chromate in solution inhibits metal dissolution and oxygen reduction reactions. It also slows metastable pitting, the transition to stable pitting, and, when present in sufficient concentration, the growth stage of pitting and crevice corrosion. 

G.T. Burstein and P C. Pistorius, Surface Roughness and the Metastable Pitting of Stainless Steel in Chloride Solutions, Corrosion, Vol 51, 1995, p 380-385... 

Several examples of the results of crevice corrosion are shown in Figs. 3 to 6. The similarities in topography amongst the examples include the accelerated attack of the substrate under the crevice former and the virtual absence of attack on the fully exposed surface. The accelerated attack within the crevice usually appears as uniform corrosion or pitting. In some cases, it is thought that the attack starts as metastable pits that coalesce into a more uniform attack. 

It is critical to avoid artifacts associated with crevice corrosion, which will swamp the metastable pitting signal. In order to get a low background current, it is necessary to use electrodes with small surface areas. Data collection must be at a high enough rate to distinguish the individual events. 

S. Krakowiak, H. Darowicki, P. Slepski, Impedance of metastable pitting corrosion, J. Electroanal. Chem. 575 (2005) 33-38. 

Fig. 7.1 Typical cyclic polarization plot for stainless steel that shows the corrosion potential, c, critical pitting potential, Epn, protection potential, Eprot, and metastable pitting region.

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