Inter Granular Corrosion, IGC

Strauss test (EN ISO 3651-2, EN 114-72, ASTM A 262 Practice E)

The samples are boiled for 15, 20 or 24 hours (depending on the standard concerned) in a solution of copper sulphate and sulphuric acid to which copper chips have been added. Evaluation takes place by bending the specimens in the manner prescribed in the standard and visual inspection of the surface in respect of cracks indicative of intergranular corrosion. Prior to this boiling, the specimens must normally undergo a sensibilizing heat treatment.

Huey test (ASTM A 262 Practice C, DIN 50921)

The specimens are boiled for five periods, each of 48 hours, in a 65 per cent solution of nitric acid. The corrosion rate during each boiling period is calculated from the decrease in the weight of the specimens. Properly interpreted, the results can reveal whether or not the steel has been heat-treated in the correct manner. The customer must specify the maximum permissible corrosion rate and, in applicable cases, data on sensibilizing heat treatment.

Pitting and Crevice Corrosion

CPT test (ASTM G 48 Method A)

The samples are exposed in a solution consisting of 100 g FeCl3, 6H20 dissolved in 900 ml of distilled water. CPT stands for Critical Pitting Temperature and is determined by increasing the temperature of the solution in steps until pitting occurs. The maximum temperature without attack is called the CPT. When this method is used for delivery testing, the customer usually prescribes the test temperature which is to be used. Inspection under a microscope, possibly supplemented with measurement of the weight loss, is used to establish whether or not an attack has occurred. If it has, the sample has failed the test. The standard specifies a testing time of 72 hours, but customers often prescribe a testing time of 24 hours, which has shown to give the same result.

CCT test (ASTM G 48 Method B)

CCT stands for Critical Crevice Temperature and the test is performed in the same way as the CPT test described above, with the difference that in this case the specimens are covered with crevices. The standard prescribes how this is to be performed in the case of sheet specimens: two teflon cylinders are clamped with rubber bands. In the case of tube methods, this method cannot be used and instead the elaboration of the crevices must be discussed with the customer in each individual case. Since crevice corrosion takes place more easily than pitting, the CCT is always lower than the CPT.