SECTION 5 

 STAINLESS STEELS 



The corrosion resistance of stainless steels is 

 attributed to a very thin, stable oxide film on the 

 surface of the alloy which results from the alloying of 

 carbon steels with chromium. Chromium, being a 

 passive metal (corrosion resistant), imparts its 

 passivity to steel when alloyed with it in amounts of 

 12% or greater. These iron-chromium alloys are very 

 corrosion-resistant in oxidizing environments because 

 the passive film is maintained in most environments 

 when a sufficient amount of oxidizing agent or 

 oxygen is present to repair any breaks in the protec- 

 tive film. 



The corrosion resistance of stainless steels is 

 further enhanced by the addition of nickel to the 

 iron-chromium alloys. This group of alloys is popu- 

 larly known as the 18-8 (18% chromium — 8% nickel) 

 stainless steels. 



In general, oxidizing conditions favor passivity 

 (corrosion resistance), while reducing conditions 

 destroy it. Chloride ions are particularly agressive in 

 destroying this passivity. 



Stainless steels usually corrode by pitting and 

 crevice corrosion in seawater. Pits begin by break- 

 down of the passive film at weak spots or at non- 

 homogeneities. The breakdown is followed by the 

 formation of an electrolytic cell, the anode of which 

 is a minute area of active metal and the cathode of 

 which is a considerable area of passive metal. The 

 large potential difference characteristic of this 

 "passive-active" cell accounts for considerable flow of 

 current with attendant rapid corrosion (pitting) at the 

 small anode. 



Pitting is most likely to occur in the presence of 

 chloride ions (for example, seawater), combined with 

 such depolarizers as oxygen or oxidizing salts. An 

 oxidizing environment is usually necessary for pre- 

 servation of passivity with accompanying high 

 corrosion resistance; but, unfortunately, it is also a 

 favorable condition for pitting. The oxidizer can 

 often act as a depolarizer for passive-active cells that 

 were established by breakdown of passivity at a 

 specific point or area. The chloride ion in particular 

 can accomplish this breakdown. 



Stainless steels can and do pit in aerated seawater 

 (near neutral chloride solution). Pitting is less pro- 

 nounced in rapidly moving seawater (aerated 

 solution) as compared with partially aerated, stagnant 

 seawater. The 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. 



As discussed above, stainless steels generally cor- 

 rode in seawater by pitting and crevice corrosion; 

 therefore, "as much as 90 to 95% of the exposed 

 surface can be uncorroded. With such low percentages 

 of the total exposed area affected, corrosion 

 calculated from loss in weight as mils penetration per 

 year (mpy) can give a very misleading picture. The 

 mpy implies 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. Tunnel corrosion is also classified by some 

 as edge, honeycomb, or underfilm corrosion. Tunnel 

 corrosion is insidious because of its nature and 

 because many times it is not apparent from the 

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

 surface or on an edge and propagates 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). This 

 type of corrosion occurs underneath deposits of any 

 kind on the metal surface, underneath barnacles, and 

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

 steel that 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 concen- 

 tration areas (the low oxygen concentration area 

 becomes the anode of the cell). 



Low weight losses and corrosion rates accompany 

 these manifestations of corrosion. Thus, the integrity 



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