Crevice 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. 

The amount of chloride, sulfate, thiosulfate, or other aggressive anions dissolved in water necessary to produce noticeable attack depends on many interrelated factors. Extraordinarily, if the water is quite aggressive, general corrosion may occur so rapidly outside the crevice that concentration differences cannot easily develop between the crevice interior and exterior. However, it is usually safe to assume that as the concentration of aggressive anions increases in solution, crevice attack is stimulated. Seawater chloride concentrations produce severe attack in most stainless crevices in a few weeks. 

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... 

Nickel is resistant to chloride-induced SCC, but subject to caustic cracking in aerated solutions under high stress. Nickel is highly resistant to corrosion in natural fresh water and flowing seawater. Pitting occurs under stagnant or crevice conditions. 

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...

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