Crevice corrosion Subject

Frequently, so-called crevice washers are used in coupon studies to test the environment for its ability to produce crevice corrosion (Fig. 2.22). There are several designs most consist of a small Teflon washer with radially oriented, wedge-shaped teeth. The washer is held to the coupon by a mounting bolt that passes through a central hole. The spaces between teeth form small crevice-shaped areas in which attack may occur (Fig. 2.23). The test is somewhat subjective and is not easily quantified. Using this test, attack in crevices either occurs or does not. 

It would not be expected that crevice corrosion could easily occur in demineralized water. However, this exchanger was subject to extended outages with no lay-up procedures. Under such conditions, crevice attack is more likely. 

The alloy Haynes 6B is resistant to corrosion in organic acids, but subject to pitting and crevice corrosion and SCC in chloride media. The corrosion rate of 0.3 mm/yr or 12mpy has been observed in 30 wt % of NaOH it is likely that caustic cracking will occur at high concentrations of NaOH and temperatures in the case of all the cobalt alloys. The nominal composition of high-temperature cobalt alloys is given in Table 4.54. 

Causes of corrosion are the subject of extensive investigation by industry. Almost any type of corrosion can manifest itself under widely differing operating conditions. Also, different types of corrosion can occur simultaneously. It is not uncommon to see crack growth from stress corrosion to be accelerated by crevice corrosion, for example. 

The rings tested at Ukrneft Co. have shown tight sealing of the tubing butts, are not subjected to paraffin deposition, reduce resistance to liquid flows and are operable in temperatures up to 100° C [148]. The rings of inhibited composites suppress crevice corrosion on pipe faces and prolong their life by 1.5-2 times. 

The bridge at the Charing Cross Railway in central London was subject to corrosion from dog s urine. Repeated urination by dogs caused crevice corrosion in a part of the structure that was not easily accessible. In 1979, the City of Westminster, London, reported a problem of falling lamp posts. The culprits were dogs that urinated at the base of the lamp posts. 

The existence of corrosion forms typical of the oil and gas industry does not mean that this industry and the related infrastructures are not potentially subject to other forms of corrosion, such as general corrosion, galvanic corrosion, corrosion under deposit, crevice corrosion, intergranular corrosion, stress corrosion, corrosion fatigue, fretting corrosion, corrosion of welds, creep and high-temperature corrosion, lUifoim corrosion, and cathodic disbondment. 

Stainless steels have very good corrosion resistance in natural water. They corrode very slowly but are subject to pitting or crevice corrosion if there is an appreciable chloride content. Further details are given in DIN 50 930, part 4 (1993). Stainless steels are also known to be susceptible to stress-corrosion cracking in chloride-containing... 

Type 317 stainless steel contains greater amounts of molybdenum, chromium, and nickel than type 316. The chemical composition is shown in Table 10.1. As a result of the increased alloying elements, these alloys offer higher resistance to pitting and crevice corrosion than type 316 in various process enviromnents encoimtered in the process industry. However, they may still be subject to chloride stress corrosion cracking. The alloy is... 

Zirconium is considerably more resistant to chloride SCC than stainless steels are. No failure was observed in U-bend tests conducted in boiling 42% magnesium chloride (MgCl2) at TWC. Another attractive property of zirconium is its high crevice corrosion resistance. Zirconium is not subject to crevice corrosion even in acidic chloride at elevated temperatures. No attack was observed on zirconium in a salt spray environment. ... 

Of all the corrosion-resistant metals and alloys, zirconium is among the most resistant to crevice corrosion. In low-pH chloride solutions or chlorine gas, for example, zirconium is not subject to crevice corrosion. This resistance can be attributed to zirconium s excellent corrosion resistance in HCl solutions. 

The conventional steels, like SS 304 and SS 316, can be subject to crevice corrosion in chloride containing environments, such as brackish water and seawater. Water chemistry pla) a very important role. 

Stainless steels. Stainless steels are not subject to impingement attack, but are prone to pitting and crevice corrosion under low-velocity conditions, and this must be taken into consideration when these alloys are used in seawater. Attempts to build seawater systems from standard grades of stainless steel, such as TVpe 316, have proved unsuccessful. In recent years, grades of stainless steel with high resistance to pitting and crevice corrosion have been developed. 

Nickel-based alloys. Nickel-based alloys such as Inconel 625, Hastelloys C-276 and C-22, and titanium are not subject to pitting or crevice corrosion in low-velocity seawater, nor do they suffer impingement attack at high velocity. However, price limits their use to special applications in seawater systems. 

H In weather-exposed steel structures subjected to permanent moisture and salt actions, so-called crevice corrosion is a general phenomenon. Crevice corrosion especially occurs in water-fiUed cracks at joints the corrosion is caused by differences in dissolved, atmospheric oxygen 02(aq) in the water. The following two electrochemical reactions are known to be inclnded in the corrosion process... 

Avoid the haphazard application of insulation and surface coverings and consider the likelihood of creating adverse corrosive conditions (chemical effect, thermal, or electrochemical imbalance) or forming crevices on the surfaces of metals subject to excessive crevice corrosion damage (e.g. stainless steels) this also applies to the apphcation of surfactants (see Figure 10.9). 

This pipe needed reinforcement at its pipe support area for the total length of 150 m due to active galvanic and crevice corrosion. This is a very common problem with racked piping. The lines are subject to movement and wear away their coatings at the contact point with the pipe rack. Recoating is impossible without lilting the pipe. Water collects in these areas, speeding up the crevice corrosion. 

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