Experimental Equipment for CPT Testing

A common chemical laboratory test for corrosion resistance is a simple exposure test using metal coupons. The ASTM standard G48 —Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride Solution—describes a simple exposure test. The material coupons (e.g., 60 x 60 mm) are placed on a glass cradle and immersed in the solution in such a way that the coupons are evenly exposed. 

In electrochemical measurements it is necessary to establish good insulated electrical contact with the sample and furthermore to have a well-defined exposed surface area. It is difficult to comply with these demands without creating, at the sample/mount interface, a crevice between the sample and the sample holder. If the bulk solution is not prevented from entering the crevice, crevice corrosion attack is often initiated.  

Crevice corrosion is another form of localized corrosion, which is easier to initiate than pitting. Since aevice corrosion will occur at lower potentials and temperatures than pitting, it is necessary to avoid crevice corrosion completely when trying to characterize resistance to pitting. Over the years, several suggestions have been presented for solving this problem. 

Bernhardsson et used argon gas to avoid crevice corrosion. 

The cylindrical specimens (exposed area 3-10 cm ) were placed vertically in the cell. The specimen holder was provided with a collar that was filled with argon during testing. The crevice between the specimen holder and specimen was filled with a stream of argon to hold back solution from the crevice. However, distribution of the gas is critical and difficult to control satisfactorily. 

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