Crevice corrosion seawater

Ma.rine. In the presence of an electrolyte, eg, seawater, aluminum and steel form a galvanic cell and corrosion takes place at the interface. Because the aluminum superstmcture is bolted to the steel bulkhead in a lap joint, crevice corrosion is masked and may remain uimoticed until replacement is required. By using transition-joint strips cut from explosion-welded clads, the corrosion problem can be eliminated. Because the transition is metaHurgicaHy bonded, there is no crevice in which the electrolyte can act and galvanic action caimot take place. Steel corrosion is confined to external surfaces where it can be detected easily and corrected by simple wire bmshing and painting. 

The stainless steels contain appreciable amounts of Cr, Ni, or both. The straight chrome steels, types 410, 416, and 430, contain about 12, 13, and 16 wt % Cr respectively. The chrome—nickel steels include type 301 (18 wt % Cr and 9 wt % Ni), type 304 (19 wt % Cr and 10 wt % Ni), and type 316 (19 wt % Cr and 12 wt % Ni). Additionally, type 316 contains 2—3 wt % Mo which gready improves resistance to crevice corrosion in seawater as well as general corrosion resistance. AH of the stainless steels offer exceptional improvement in atmospheric conditions. The corrosion resistance results from the formation of a passive film and, for this reason, these materials are susceptible to pitting corrosion and to crevice corrosion. For example, type 304 stainless has very good resistance to moving seawater but does pit in stagnant seawater. 

Cathodic protection applications in fresh water include use of ferrite-coated niobium , and the more usual platinum-coated niobium . Platinised niobium anodes have been used in seawater, underground and in deep wells " and niobium connectors have been used for joining current leads Excellent service has been reported in open-seawater, where anodic potentials of up to 120V are not deleterious, but crevice corrosion can occur at 20 to 40V due to local surface damage, impurities such as copper and iron, and under deposits or in mud ... 

Guide for crevice corrosion testing of iron base and nickel base stainless steels in seawater and other chloride-containing aqueous environments... 

Guide for Crevice Corrosion Testing of Iron-Base and Nickel-Base Stainless Alloys in Seawater and Other Chloride-Containing Aqueous Environments, G 78, Annual Book of ASTMStandards, ASTM, 1992, p 463-470... 

Corrosion of Titanium in Neutral and Alkaline Solutions In water, steam, and seawater, titanium is resistant even at high temperatures [44]. In water with high chloride levels, crevice corrosion could appear if tight crevices are present. Titanium shows very low corrosion rates even in seawater [43],... 

Fig. 2 Crevice corrosion under seal in Type SS316 sieve from steam condenser cooling water system exposed to flowing seawater for 2 years at less than 40 °C [1].

Ignored by most implementations of the CCS framework, ohmic drop can not only lead to passive-to-active transitions, but also can, in the context of environmental cracking, make hydrogen evolution, and therefore embrittlement, more viable at the crack tip. The IR framework has been successfully demonstrated in several model metal/environment systems [34, 35], and has been invoked to rationalize the practically important case of the crevice corrosion of Alloy 625 in chlorinated seawater [32, 33]. 

T.P. May, H. Humble, Effectiveness of cathodic currents in reducing crevice corrosion of several materials in seawater. Corrosion 8 (1952) 50-56. 

Figure 7.18 Prediction of whether or not crevice corrosion will occur in crevices 5 mm deep in seawater at ambient temperature as a function of crevice gap. (After Oldfield and Todd [7.14].)...

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.

If, on the other hand, a less noble metal with free surface is connected to a more noble component with a crevice, the coupling will counteract crevice corrosion in the latter. Certainly, in many cases of stainless steel exposed to seawater, crevice corrosion is prevented by contact with unalloyed/low-alloy steel or cast iron. An example is stainless steel pumps or valves coupled to ordinary steel or iron pipes. 

Selection of material. As dealt with in previous sections, conventional stainless steels, with martensitic, ferritic, austenitic or ferritic-austenitic (duplex) structure, are sensitive to crevice corrosion (Table 7.4). Newer high-alloy steels with high Mo content show by far better crevice corrosion properties in seawater and other Cl-containing environments (see Section 10.1). 

Table 7.4 Resistance of various materials to crevice corrosion in stagnant seawater...

The latter steels have been developed during the last few decades. (For compositions, properties and standard numbers, see Table 10.6.) A benefit of the ferritic-austenitic steels is their higher strength compared with the austenitic. Ferritic-austenitic 25-7-4 steel and austenitic 20-18-6 have both shown very good crevice corrosion properties in seawater, but also on these steels attacks may develop when the temperature is above a limit that depends on various conditions. Of these two steel types, the ferritic-austenitic steel may have somewhat lower corrosion resistance in welds than the austenitic 6 Mo steel. With first-class welds, pipes made of the latter material are considered safe to use up to 30-35°C in seawater. However, on flanges that are cast or produced by powder metallurgy, attacks have been found at a temperature as low as 10-15°C. For use in chlorinated seawater with a residual chlorine content of 1.5 ppm, the NORSOK standard [10.10] recommends a maximum temperature of 15°C for components with crevices and 30°C for... 

Open circuit potential As defined by the McGraw-Hill Dictionary of Scientific and Technical Terms, open circuit potential (OCP) is the steady-state or equilibrium potential of an electrode in absence of external current flow to or from the electrode. OCP measures the corrosion potential of a corroding metal with regard to a reference electrode. For instance, increased susceptibility of stainless steels to pitting and crevice corrosion in seawater has been attributed to increase in OCP, which could partly be due to biofllm formation. Monitoring of OCP spectra can be used to rank the corrosion vulnerability of metals in comparison with each other. 

Differential aeration cells can also lead to localized corrosion at pits (crevice corrosion) in stainless steels, aluminum, nickel, and other passive metals that are exposed to aqueous environments, such as seawater. 

Stainless steels exposed to seawater develop deep pits within a matter of months, with the pits usually initiating at crevices or other areas of stagnant electrolyte (crevice corrosion). Susceptibility to pitting and crevice corrosion is greater in the martensitic and ferritic steels than in the austenitic steels it decreases in the latter alloys as the nickel content increases. The austenitic 18-8 alloys containing molybdenum (types 316,316L, 317) are still more resistant to seawater however, crevice corrosion and pitting of these alloys eventually develop within a period of 1-2.5 years. 

By alloying nickel with both molybdenum and chromium, an alloy is obtained resistant to oxidizing media imparted by alloyed chromium, as well as to reducing media imparted by molybdenum. One such alloy, which also contains a few percent iron and tungsten (AUoy C), is immune to pitting and crevice corrosion in seawater (10-year exposure) and does not tarnish appreciably when exposed to marine atmospheres. Alloys of this kind, however, despite improved resistance to Cl, corrode more rapidly in hydrochloric acid than do the nickel-molybdenum alloys that do not contain chromium. 

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