Incubation corrosion

There is often a period before corrosion starts in a crevice in passivating metals. This so-called incubation period corresponds to the time necessary to establish a crevice environment aggressive enough to dissolve the passive oxide layer. The incubation period is well known in stainless steels exposed to waters containing chloride. After a time period in which crevice corrosion is negligible, attack begins, and the rate of metal loss increases (Fig. 2.8). 

Figure 2.8 Schematic representation of corrosion rate as a function of time in a crevice in stainless steel exposed to chloride-containing water. The time before corrosion initiation is called the incubation period.

The anodically produced acid is neutralized by the alkaline mortar (CaO). Corrosion is then possible only if the supply of alkali at the steel surface is consumed and the steel becomes active. This process is possible only under certain circumstances after a very long incubation period. Apparently in steel-concrete foundations the possible current densities are so small that this case never arises. The possibility of danger has to be verified with thin outer coatings where deliming has been noticed on the steel surface. 

Crolet, J. and Defranoux, J.M., Calcul du Temps d Incubation de la Corrosion Caverneuse des Aciers Inoxydables , Corros. Sci., 13, 575 (1973)... 

Any test (several such tests are used) in which time to failure of smooth specimens is determined is an overall measure of the incubation period to initiate a crack, the ability to resist the propagation of a stress corrosion crack and the ability to resist final mechanical fracture. Since this test does not indicate the relative merits of an alloy in each individual aspect of the... 

When corrosion of a chromium-coated metal takes place, the corroding current concentrates its action on fissures in the deposit. There appears to be an incubation period, after which rapid attack occurs in the form of pits, and... 

S.C.C. has received a share of the potentiostatic approach to corrosion. Barnartt and van Rooyen reported that potentiostatically controlled corrosion in a potential range 50-100 mV above the corrosion potential provided an accelerated test for the s.c.c. of stainless steels. The elevation of the potential by means of a potentiostat eliminated the incubation period, and also increased the density of cracks. Booth and Tucker used potentiostatic methods in the s.c.c. of Al-Mg alloys. 

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.

Data should generally be for a test conducted for as long as practical but always for a minimum of 300-500 hours or more. Short time tests may mask incubation periods for attack on interior bond phases which might differ in susceptibility to corrosion as compared with the bond phases of the outer surfaces of parts, especially in SisN4, Si20N2and reaction bonded products. 

Corrosion of concrete by bacteria that oxidize sulfur compounds into sulfuric acid was reported by Parker (1945, 1947) and has been examined more recently by Fjerdingstad (1969). The lowest pH is tolerated by Thiobacillus concretivorus Parker (pH 2.75 + 0.42) and T. thiooxidans Waksman and Joffe (pH 2.35 0.32). Within 100 days of incubation in a T. thiooxidans culture, 60% of a 3.6 g block of concrete were dissolved. Bacterial destructive activity has also been established on buildings and monuments (Krum-... 

The current results of 5000 hr tests at 1800°F in the CGA gas containing 0.5 v/o H2S are summarized in Table IV. Here the 1000, 3000, and 5000 hr total corrosion data have been linearly extrapolated to mpy corrosion rates. Several alloys--for example, AISI 309, AISI 310, and AISI 314--that had extrapolated corrosion rates of 20-40 mpy after 1000 hr indicated transitions to higher rates in 1000-2000 hr. As a result, the linearly extrapolated rates at 3000 hr were greater than 80 mpy. Other alloys, such as HK-40 and IN-617, exhibited internal penetration at 1000 hr that did not increase significantly for the longer exposures. For these alloys, corrosion data obtained by linear extrapolation of 1000 hr data was maintained for 3000 and 5000 hr, respectively. The data shown in Table IV illustrate that 1000 hr corrosion data could not be linearly extrapolated to mpy corrosion rates for all alloys. It was necessary to conduct 5000 hr tests to verify the existence of incubation times of 1000-4000 hr for transitions to rapid corrosion. 

The sample shown in Fig. 3 was held several hundred millivolts below its pitting potential [28]. After an incubation period, crevice corrosion initiated. The area immediately inside the crevice was... 

Heavy metals, such as chromium, that are toxic to certain microbes may increase production of EPS. Fang and colleagues [101] found an increase in the EPS of an SRB-enriched marine culture when chromium (50-100 ppm) was added to seawater. Exposure of mild steel samples to this solution for 20 d under anaerobic conditions resulted in corrosion. A subsequent study by Chan and coworkers [102] suggested that corrosion of mild steel, immersed in synthetic seawater containing extracted and purified EPS (from the same SRB-enriched culture as above) and incubated under anaerobic conditions as before, was aided by oxidation reactions provided by EPS. The electrochemical reductions of EPS were coupled to iron oxidation. Polysaccharides in EPS were electrochemically reduced and converted to hydrocarbons, as shown by changes in the X-ray photoelectron spectra. 

The procedure for determining a current density-potential curve is to immerse the metal electrode in the corrosive medium. After an incubation period, the mixed potential corresponding to the outwardly currentless state becomes established on the metal. If a circuit is now placed on the electrode and external current is applied, the result is a shift in potential polarization. Current density-potential curves are therefore also known as polarization curves. Depending on the direction of the external current imposed, this is termed anodic or cathodic polarization, and anodic potential becomes positive while cathodic potential becomes negative. 

It should be noted that small variations in the linear loss rate constant give macroscopically different behavior the curve labeled data 1 in Fig. 7 might wrongly be interpreted as hardly any change after a short period and data T as active corrosion after an incubation period . 

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