Crevice corrosion stages

Figure 2.4 Crevice corrosion—initial stage in oxygenated water containing sodium chloride. (Courtesy of Mars G. Fontana and Norbert D. Greene, Corrosion Engineering, McGraw-Hill Book Company, New York City, 1967.)...

Since pp of the metal within the crevice is raised by the decrease in pH, the metal is active whereas the outer surface is passive, and this represents the active propagation stage of crevice corrosion there is a significant IR drop between the two zones. 

Crevice corrosion may occur where liquid is trapped between close fitting metal surfaces, or between a metal surface and a non-metallic material such as a gasket. Attention to detail at the design and fabrication stage should be given to areas such as jointing to prevent crevice corrosion. 

Chromates are particularly effective inhibitors, and there appear to be several components to inhibition. Chromate in solution inhibits metal dissolution and oxygen reduction reactions. It also slows metastable pitting, the transition to stable pitting, and, when present in sufficient concentration, the growth stage of pitting and crevice corrosion. 

Figure 7.15 shows two stages in the development of crevice corrosion. A flange joint of a metal M is exposed to neutral water containing oxygen and NaCl. For simplicity, we assume firstly that M is a univalent metal. The development can be divided into the following four stages ...

Figure 7.15 Crevice corrosion, a) Initial and b) later stage. (From Fontana and Greene [7.1].) Reproduced with the permission of The McGraw-Flill Companies.

Figure 7.16 Potential-log current diagram for crevice corrosion, a) Initial and b) developed to a late stage. 1) Anodic and 2) cathodic curve (oxygen reduction) in neutral water. 3) Anodic curve in HCl solution, pH = 0, corresponding to a crevice solution. 4) Curve for the hydrogen reaction in the crevice (pH = 0-4). 5) Curve for the reduction of oxygen outside the crevice (pH = 8-10).

FIGURE 14.12. Schematic depicting the initial and later stages of crevice corrosion. (With permission from NACE International.)... 

Corrosion of a substrate beneath an organic coating is an electrochemical process which follows the same principles as corrosion of an uncoated substrate. It differs from crevice corrosion since the reactants often reach the substrate through a solid. In addition, during the early stages of corrosion small volumes of liquid are present, resulting in extreme values of pH and ion concentrations. The total corrosion process takes place as follows ... 

Lee, T. S., A Method for Quantifying the Initiation and Propagation Stages of Crevice Corrosion, Electrochemical Corrosion Testing, ASTM STP 727, F. Mansfield and U. Bertocci, Eds., ASTM International, West Conshohocken, PA, 1981, pp. 43-68. 

The series of events leading to the formation of a severe crevice can be summarized in the following three stages. Firstly, crevice corrosion is believed to initiate as the result of the differential aeration mechanism mentioned earlier. IDissolved oxygen in the liquid which is deep in the crevice is consumed by reaction with the metal [Fig. 6.21( )]. Secondly, as oxygen diffusion into the crevice is restricted, a differential aeration cell tends to be set up between the... 

Crevice corrosion is ideally prevented at the design stage by avoiding them in the first place or during construction by filling uncorroded dry crevices with a durable jointing compound that will exclude moisture and remain resilient. 

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