Stainless steels, nickel-base and chromium-bearing alloys may be subjected to crevice and critical crevice temperature investigation. Two TFE-fluorocarbon blocks are fastened with O-rings or rubber bands to the test specimens. This operation is made in order to simulate a stagnant microenvironment in which crevice tends to occur. Then, test pieces are immersed in a ferric chloride solution (FeCl3) at constant temperature for a specified time. After this, specimes are rinsed, brushed in order to remove corrosion products, weighed and examined. Following which, specimen mass loss is calculated and the number of attacked sites, together with the maximum and average depth of attack under the TFE-fluorocarbon blocks and at the point of contact for the O-rings or rubber bands are recorded.
The ASTM G 48 standard test method provides the procedures to detect the critical temperature at which crevice corrosion initiates. The starting temperature is estimated by an equation multiplying chrome, nickel and molybdenum percentages and standard coefficients. If 0,025 mm or deeper crevices start at this temperature, the test is repeated at a lower temperature; otherwise the test is repeated at a higher temperature. This method allows to detect the actual critical crevice temperature of the material.
Stainless steels and nickel and chromium-based alloys are normally subjected to crevice tests. The metal samples are tied to Teflon blocks by means of rubber bands also made of this material. The aim is to create areas of stagnation potentially subject to the phenomenon described above. After having immersed them in a solution of ferric chloride (FeCl3) at a constant temperature and for a given duration, the samples are rinsed, weighed and examined. The loss of mass, the average and maximum depth of the crevices near the contact points between the metal surface and the applied elastomers are therefore evaluated.
The ASTM G 48 standard also defines the methods for identifying the temperature at which the corrosion process described above is triggered. The starting temperature is estimated according to a formula which multiplies the percentages of Chromium, Nickel and Molybdenum by established coefficients. If at this temperature, also known as the Critical Crevice Temperature, crevice are noted with a depth greater than or equal to 0.025 mm, the test is repeated by lowering the temperature; if, on the other hand, crevices are not as deep, the test is repeated by raising it. In this way it is possible to identify the effective critical ignition temperature of the crevice. The ASTM G 48 standard also defines the methods for identifying the temperature at which the corrosion process described above is triggered.
- ASTM G 48 – Method B
- ASTM G 48 – Method D
- ASTM G 48 – Method F
The term ” crevice ” refers to a localized form of corrosion usually associated with a stagnant solution on the micro-environmental level.
Interstitial areas act as an anod, causing the reduction of iron, while surrounding areas induce cathodic reactions, i.e. the reduction of oxygen and water. Crevice mechanism tends to occur in areas included between two metals, a metal and a non-metallic materials (e.g. ceramic) or a metal and non-metallic deposits (e.g. silicates).
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