Crevice corrosion initiation time

Biofilms on passive alloy surfaces can increase cathodic kinetics by way of increasing the propagation rate of galvanic corrosion. Cathodic kinetics increased during biofilm formation on passive alloy surfaces. Crevice corrosion initiation times were reduced when natural biofilms were allowed to form on passive alloys S 30400 and S 31600. 

The production of aqueous sulfide via the chemical dissolution of MnS inclusions was shown to increase the aggressiveness of the low pH, high [Cl ] occluded solution as shown in Fig. 10 such that crevice corrosion can occur at rates on the order of 30 to 50 mA cm . As the alloy sulfur content decreases, the difficulty in obtaining the aqueous sulfide content increases, lengthening the initiation time. Nonetheless, even at the lowest alloy sulfur contents studied (17 ppmw), crevice corrosion initiation due to aqueous sulfide accumulation still occurred. 

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 rate of metal wastage of this indirect form of corrosion may be increased by the presence of other direct corrosion influences in the deposit or foulant. Also (and similar to crevice corrosion), there may be general oxygen corrosion occurring at the same time or perhaps acting as an initiator to the under-deposit corrosion process. 

The presented data is a confirmation that it is expedient to use oil-soluble Cl as components of polymer coatings. If their concentration is optimal, the strength of adhesive joints coating metal ware can improve by 10-20%. The inhibited coatings are more resistant than those made of initial polymers. The crevice corrosion rate of the metal substrate recorded by the polarization resistance method presented in [80] lowers on transfer from the initial polyethylene coating to an inhibited one by three to ten times [81]. 

The lack of any effect of sulfate on R indicates that the mechanism by which sulfate is inhibiting crevice corrosion is through a supporting electrolyte effect [51-54]. Thus, the increases in initiation time are due to the competitive migration of S04 and Cl , with accumulation of the critical concentration of Cl needed being... 

The addition of nitrate to the bulk solution led to more significant delays in initiation of crevice corrosion (Fig. 7) of 304 SS compared to sulfate. For example, to produce an initiation time of 75 min required 1.5 mM S04 , but only 23 pM NOs. At low bulk [Cl ] [NO3-] ratios ([N03 ] = 1.7 mM), no initiation of crevice corrosion was observed within 180 min, whereas in solutions in which crevice corrosion did occur the composition of the crevice solution was essentially independent of the initial concentration of nitrate in the bulk solution. However, the relative concentration of Cl compared to NO3 in the crevice was nearly three times... 

In one of the tests, the corrosion potential of the sample in 9 g/1 sodium chloride is noted. The pH, initial, and final potential are noted. The current at -1-0.8 V versus SCE is noted. If localized corrosion is not simulated in the first 20 s, the polarizing currents will be small and decrease with time. If localized corrosion is not stimulated in 15 min, the test is terminated, and the sample is resistant to localized corrosion. Localized corrosion is indicated by increasing polarizing current with time (>500 pA/cm ). The potential is then returned to corrosion potential to determine if the sample will repassivate or the localized corrosion will continue to occur. Evidence of pitting and crevice corrosion should be noted in ASTM F 746 (30). 

Figure 7.23 Potential as a function of time for various stainless steels in seawater at a flow velocity of 1.2 m/s and temperature 9 C. The potential drop after about 20 days for one of the materials is due to initiation of crevice corrosion.

The initial corrosion racks provided by the IAEA were immersed in late 1996 in water storage pools with a wide range of water chemistry and environmental conditions, and were monitored for corrosion over a period of time. The results of these early observations were reported after 18 months at the second research co-ordination meeting (RCM) of the CRP, held in Sao Paulo, Brazil. Pitting and crevice corrosion were the primary corrosion mechanisms observed. Corrosion by deposition of iron and other cathodic particles on the... 

The remote crevice assembly technique (see Chapter 19) is a research tool that allows one to separate the anode and cathode areas of a crevice corrosion test sample so that the current flowing between them can be measured with a zero-resistance ammeter. This technique is similar to the dual cell method, and it lends itself well to studies of microbial effects on crevice corrosion [7]. It allows direct measurement of microbial effects on both the initiation time and propagation rate for crevice attack, provided again that a suitable control experiment without the microbial influence can be done concurrently. The scime technique of separating the anode and cathode can be used to study the influence of microbes in biofilms on galvanic corrosion [li]. 

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