Crevice corrosion stainless steels

Pretreatments of Stainless Steels - Crevice Corrosion and High Temperature Oxidation (AES)... 

In ferritic and austenitic stainless steels, crevice corrosion is almost always initiated by local activation. This can be induced in a crevice by oxidant depletion, if necessary supplemented by halides. The passivity then breaks down. The access of oxidants to the material surface, and hence the passivity, may also be hindered by local deposits. 

The crevice shape markedly affects corrosion. Crevices so tight that water may not enter are entirely immune to attack. In misting environments or alternately wet-diy environments, the crevice holds water and may allow continued attack even when neeu by surfaces eire dry. In sea water, the severity of attack in stainless steel crevices depends on the ratio of the crevice area to the cathodic surface area outside the crevice. If the cathodic area is large relative to crevice eirea, corrosion is promoted. 

Corrosion products are almost always absent in stainless steel crevices. Areas just outside stainless crevices are stained brown and orange with oxides (Figs. 2.20 and 2.21). Metal ions migrate out of the crevice. Precipitation occurs by reactions similar to Reactions 2.3 and 2.4. Crevice interiors remain relatively free of rust (Figs. 2.16 and 2.17). 

Access of air and water will also affect the corrosion rate. Metal inserts in corrosive plastics are most actively attacked at the plastic/metal/air interfaces with certain metals, notably aluminium titaniumand stainless steel, crevice effects (oxygen shielding and entrapment of water) frequently accelerate attack. Acceleration of corrosion by bimetallic couples between carbon-fibre-reinforced plastics and metals presents a problem in the use of these composites. 

Oxide stabilized refers to materials, such as aluminum and the stainless steels, whose corrosion resistance depends on the formation and stability of a very thin surface oxide layer that is inert, easily healed if damaged, and tenacious. When the oxide layer has been disrupted and not healed, the material usually has little corrosion resistance. Both active and passive states sometimes exist adjacent to each other on the surface, resulting in rapid local corrosion. Crevice corrosion in stainless... 

For crevices such as in those in socket welds, the metal in the crevice is likely to be anodic. Crevice corrosion and under-deposit corrosion can be serious problems in oxide-stabilized materials such as aluminum and the stainless steels. Crevices and deposits can also accelerate corrosion in metals (such as carbon steel) that do not exhibit both active and passive states. However, the rate of corrosion is much slower in such materials because they lack the galvanic driving force of the active-passive states characteristic of the oxide-stabilized metals and alloys. The anode areas in crevices and under deposits are typically smaller than the cathode areas. This difference accelerates the corrosion rate. 

Under many other conditions these are mueh higher. The group named weld corrosion in the tabic is often a kind of galvanic corrosion. On stainless steel, weld corrosion may in many cases mean crevice corrosion in or at the weld. 

Lizlovs, E. A., Crevice Corrosion of Some High Purity Ferritic Stainless Steels, Localized Corrosion—Cause of Metal Failure, ASTM STP 516, ASTM International, West Conshohocken, PA, 1972, pp. 201-209. 

Stainless Steel Type Corrosion Rate mg do 2/day Deepest Penetration in Creviced Area, microns Remarks... 

The initiation of crevice corrosion by pitting inside the crevice gap is mainly invoked for stainless steels. Indeed, several investigators [23,46,63,64] have observed pitting and further coalescence of the pits inside stainless steel crevices. Eklund [64] showed that pitting on MnS inclusions near the mouth of the crevice may be the initial stage of crevice corrosion of stainless steels. Oldfield and Sutton [46] have given a detailed description of... 

Crevice Corrosion. Crevice corrosion is intense locali2ed corrosion that occurs within a crevice or any area that is shielded from the bulk environment. Solutions within a crevice are similar to solutions within a pit in that they are highly concentrated and acidic. Because the mechanisms of corrosion in the two processes are virtually identical, conditions that promote pitting also promote crevice corrosion. Alloys that depend on oxide films for protection (eg, stainless steel and aluminum) are highly susceptible to crevice attack because the films are destroyed by high chloride ion concentrations and low pH. This is also tme of protective films induced by anodic inhibitors. 

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. 

These alloys have extensive applications in sulfuric acid systems. Because of their increased nickefand molybdenum contents they are more tolerant of chloride-ion contamination than standard stainless steels. The nickel content decreases the risk of stress-corrosion cracking molybdenum improves resistance to crevice corrosion and pitting. 

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.

Acidic pH helps break down protective oxides on stainless steels. Corrosion usually develops faster and is more severe as pH decreases. At very low pH, however, attack inside crevices may be no more severe than on regions outside the crevice. 

Figure 15.9 Cross section of stainless steel weld showing crevice corrosion along a site of incomplete fusion. (Magnification 15x.)...

In all cases partial or total hulls of aluminum or stainless steel must be provided with cathodic protection. This also applies to high-alloy steels with over 20% chromium and 3% molybdenum since they are prone to crevice corrosion underneath the coatings. The design of cathodic protection must involve the particular conditions and is not gone into further here. 

Crevice corrosion of copper alloys is similar in principle to that of stainless steels, but a differential metal ion concentration cell (Figure 53.4(b)) is set up in place of the differential oxygen concentration cell. The copper in the crevice is corroded, forming Cu ions. These diffuse out of the crevice, to maintain overall electrical neutrality, and are oxidized to Cu ions. These are strongly oxidizing and constitute the cathodic agent, being reduced to Cu ions at the cathodic site outside the crevice. Acidification of the crevice solution does not occur in this system. 

Copper has excellent resistance to some corrosive environments, including fresh waters and fluoride-containing atmospheres. Alloying is necessary to achieve good strength, but copper limiting with steel for strength is an alternative (BS 5624). Copper and some of its alloys are susceptible to crevice corrosion, but the mechanism is different from that which affects stainless steels. 

Stainless steels are particularly prone to crevice corrosion, and even the Fe-18Cr-8Ni-3Mo type of austenitic stainless steel, which is highly resistant to pitting when the surface is free from crevices, is susceptible although initiation of attack may take 1-2 years... 

Ulanovskii, I. B., Conditions for Cathodic Protection of Stainless Steels in Crevices , Zashchita Metal, 1, 643 (1965) C.A., 64, 10751 Holmes, D. R. and Mann, G. M. W., A Critical Survey of Possible Factors Contributing to Internal Boiler Corrosion , Corrosion, 21, 370 (1965)... 

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