How to measure pitting corrosion

There are many different approaches to the measurement of corrosion, and specifically pitting corrosion. A number of the industry-standard tests for corrosion, such ASTM G28 and ASTM G48, exist.

The ASTM G48 set of tests includes a number specifically configured to examine pitting corrosion resistance. In this test, coupons of the metal are immersed in a concentrated ferric chloride solution held at a specific temperature for up to 72 hours. In Test Method A, the weight loss is reported and compared with a pass-fail criteria. Ferralium 255 achieves a guaranteed weight loss of <0.8g/m2, whereas the standard pass-fail limit for S32760 (F55, 1.4501, Zeron 100) and S32750 (SAF2507, F53, 1.4410) is 4.0g/m2. This is the most common approach to comparing pitting corrosion resistance.

ASTM G48 Test Method B introduces an artificial crevice corrosion former on to the sample surface. Test Methods C to F calculate the critical pitting or critical crevice temperature for stainless steels and nickel alloys – which is the temperature above which a significant degree of corrosion will be expected to occur.

In the ASTM G28 test, the focus is more on examining intergranular corrosion rather than pitting corrosion. The test was designed to identify poor quality products, where incorrect solution annealing and quenching could result in metallic precipitates near the grain boundaries that reduce local corrosion resistance. Samples are immersed in a particularly aggressive solution of 50% boiling ferric sulphate and 50% sulphuric acid. Weight loss is compared with a reference value and used as a pass-fail criteria.

When considering development projects to improve the pitting corrosion performance of new alloys, more refined measurement techniques may be employed. Instead of focusing on weight loss alone, the number of pits, the size and depth of pits and the surface distribution of pits may all be considered to add greater insight to the susceptibility of corrosion.

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