Corrosion Testing

Corrosion testing is the collective name for the destructive testing methods that objectively measure the resistance of metallic materials and protective coatings to the electrochemical reactions they undergo with their surrounding environment (moisture, salt, acid, oxygen, temperature). Corrosion is fundamentally an oxidation–reduction (anode–cathode) process; in the micro-galvanic cells that form on the metal surface, metal ions dissolve and the material loses mass and section. This degradation does not always progress uniformly; most engineering failures originate from localised and insidious mechanisms such as pitting, crevice corrosion, intergranular corrosion or stress corrosion cracking. Corrosion tests trigger these mechanisms under accelerated and controlled laboratory conditions and thereby generate predictive data on the material's actual service life.

Some of the methods are based on accelerated atmospheric (cabinet) tests and others on laboratory immersion tests. In the salt spray (salt fog) test, specimens are exposed in a temperature-controlled cabinet to a continuous fog of a neutral sodium chloride solution of a specified concentration; on coated parts, the time until the first red or white rust appears is measured. The acetic acid salt spray (AASS) and copper-accelerated CASS tests, which are more aggressive and used for automotive and decorative coatings, add copper chloride and acetic acid to accelerate degradation many times over, so that a few days of testing simulate years of atmospheric exposure. In these tests the solution pH, cabinet temperature, fog collection rate and spray pressure are strictly fixed by the standards.

In stainless steels, nickel alloys and duplex steels the electrochemical behaviour depends on the microstructure and thermal history of the material. When chromium carbide precipitation occurs at the grain boundaries as a result of welding or incorrect heat treatment (sensitization), the surrounding region becomes depleted in chromium and susceptible to intergranular corrosion. This susceptibility is detected by boiling the specimen for a specified period in boiling copper sulphate–sulphuric acid (Strauss) or iron(III) sulphate–sulphuric acid solution and then evaluating it by bending or by metallographic cross-section examination. For pitting and crevice corrosion resistance, the iron(III) chloride immersion test is applied; in duplex steels, the presence of detrimental intermetallic phases such as sigma and chi is revealed by impact- or corrosion-based accept/reject tests.

The value of a corrosion test lies in results that are quantitative and repeatable. The evaluation measures the mass loss of the specimens before and after the test (converted into a corrosion rate in mg/cm²·day or mm/year), pit depth and density, the time to first rust, the depth of grains affected by intergranular attack, and blistering or peeling of the coating. Acceptance criteria are set by the material specification, the relevant standard or the customer/project specification; results are documented with photographs, metallographic images and measurement tables. The manufacturer is thus able to verify material selection, welding procedure and heat treatment parameters on an evidence-based footing.

TÜV AUSTRIA SILA KALİTE performs its corrosion testing services within the scope of TS EN ISO/IEC 17025 accreditation, with calibrated cabinet and laboratory equipment and under the conditions required by the relevant national and international standards. This accredited approach ensures that the corrosion resistance data produced rests on a traceable technical basis that is recognised both nationally and internationally and that can be defended before courts and auditors.