flow of seawater carries away corrosion products which would otherwise 

 accumulate at crevices or cracks. It also insures uniform passivity 

 through free access of dissolved oxygen. 



The chemical compositions of the stainless steels in this investi- 

 gation are given in Table 2. 



CORROSION 



As discussed above, stainless steels generally corrode by pitting, 

 particularly in seawater; therefore, as much as 90 to 95 percent of the 

 exposed surface can be uncorroded. Pitting typical of that usually 

 found in stainless steels is shown in Figure 3. With such low percent- 

 ages of the total exposed area affected, corrosion calculated from loss 

 in weight as mils per year (MPY) can give a very misleading picture. 

 MPY infers a uniform decrease in thickness, which for stainless steels 

 is not the case. 



A manifestation of pitting corrosion whose presence and extent is 

 often overlooked is tunnel corrosion. An illustration of tunnel cor- 

 rosion extending almost the entire length of a 12-inch specimen is 

 shown in Figure 4. Tunnel corrosion is also classified by some as edge, 

 honeycomb or underfilm corrosion. Tunnel corrosion is insidious because 

 of its nature and by virtue of the fact that many times it is not ap- 

 parent from the outside surfaces of the object. It starts as a pit on 

 the surface or on an edge and propogates laterally through the material, 

 many times leaving thin films of uncorroded metal on the exposed surfaces. 



Another manifestation of localized attack in stainless steels is 

 oxygen concentration cell corrosion in crevices (usually known as 

 crevice corrosion) . An illustration typical of crevice corrosion in 

 stainless steels is shown in Figure 5. This type of corrosion occurs 

 underneath deposits of any kind on the metal surface, underneath bar- 

 nacles and at the faying surfaces of a joint. The area of stainless 

 steel which is shielded from the surrounding solution becomes deficient 

 in oxygen, thus creating a difference in oxygen concentration between 

 the shielded and unshielded areas. An electrolytic cell is created 

 with a difference of potential being generated between the high and low 

 oxygen concentration areas with the low oxygen concentration area be- 

 coming the anode of the cell. 



Low weight losses and corrosion rates accompany these manifesta- 

 tions of corrosion. Thus, the integrity of a stainless steel structure 

 can be jeopardized if designed solely on the basis of corrosion rates 

 calculated from weight losses rather than on the basis of measured depths 

 of pits, lengths of tunnel corrosion and crevice corrosion. Pitting, 

 tunneling and crevice corrosion can, and do, penetrate stainless steel 

 rapidly, thus rendering it useless in short periods of time. 



Therefore, corrosion rates expressed as MPY calculated from weight 

 losses, maximum pit depths, maximum lengths of tunnel corrosion, and 



