Pitting corrosion is most commonly measured using industry-standard immersion tests, principally ASTM G48 for pitting and ASTM G28 for intergranular corrosion. These expose coupons of the metal to an aggressive solution and compare the result against a pass-fail criterion.
ASTM G48
The ASTM G48 set of tests includes methods configured specifically to examine pitting corrosion resistance. Coupons are immersed in a concentrated ferric chloride solution held at a set temperature for up to 72 hours.
In Test Method A, the weight loss is reported and compared with a pass-fail criterion. 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 way to compare pitting corrosion resistance.
Test Method B introduces an artificial crevice-corrosion former onto 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 significant corrosion is expected.
ASTM G28
ASTM G28 focuses on intergranular corrosion rather than pitting. It was designed to identify poor quality product, where incorrect solution annealing and quenching can leave 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, and the weight loss is compared with a reference value as a pass-fail criterion.
Measuring in development work
When developing alloys with improved pitting performance, more refined techniques may be used. Instead of weight loss alone, the number of pits, their size and depth, and their surface distribution may all be considered to give greater insight into corrosion susceptibility.
| Grade | ASTM G48A weight loss limit |
|---|---|
| Ferralium® 255 | <0.8 g/m² |
| S32760 (Zeron 100) | <4.0 g/m² |
| S32750 (SAF2507) | <4.0 g/m² |