ASTM G48 Pitting Resistance Testing

ASTM G48 Test Method for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by use of Ferric Chloride Solution is a standard testing procedure to determine the resistance of stainless steel to pitting and crevice corrosion by exposing the test pieces to oxidising solution (FeCl3).
The method is widely used for pre-qualification of stainless steel welds However, the testing method itself didn’t define the pass criteria. And the described methods leave some details to decision by the test house, details that may have decisive influence on the results. This is often been left with individual laboratory’s testing procedure if the corrosion specialist is not involved. For ASTM G48 method, although metal loss is measure, it has less implication on the general corrosion rate. T he metal loss is rather an indicator of the pitting initiation than the general metal thinning. The results may be used for ranking alloys in order of increasing resistance to pitting and crevice corrosion initiation under the specific conditions of these methods, which may result in failure through cracking.
The accuracy of ASTM G48 test method is affected by multiple factors:

ASTM G48 test piece

a. Sample location
b. Test piece side
c. Test piece preparation
d. Surface finish of the sample
e. Chemistry of test solution
Since sample and preparation method is not clearly stated in most of laboratory testing procedure and unstable nature of testing solution FeCl3, it is difficult to perform the inter-comparison for the ASTM G48 results.
The actual sampling place is important in ASTM G48 test. Some laboratories allow samples to be taken randomly, for example, in one case, “The samples were cut form duplicate 1.5 cm x 1.5 cmx 0.5 cm sub-samples of each welded parent material, removed from adjacent the root run of the welded 250NB xSCH80 pipe-to-elbow butt weld.” From this statement, the test was against the parent materials rather than the root-run and HAZ, which is the place of interest for this test
To improve the accuracy and reproducibility of the ASTM G48, TWI published its modified version ASTM G48 method as a testing method and acceptance criteria. The main clarification and improvement for this modification are:
1) The test specimen shall have a dimension of full wall thickness by 25 mm along the weld and 50 mm across the weld. The test shall expose the external and internal surface and a cross section surface including the weld zone in full wall thickness
2) The test specimen should have sides prepared to a 1200 grit finish and the test face should be as-prepared for service.
3) The test solution should be made with reagent grade chemicals, controlled purity water and an addition of Na2EDTA.2H2O.
4) The test should be assessed primarily on the basis of weight loss, with ≥20mg constituting failure. A visual examination was appropriate and that visual identification of pits should also represent failure.
Reference
1. ASTM G48 Test Method for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by use of Ferric Chloride Solution,
2. TWI, Ferric Chloride testing for weld procedure qualification of duplex stainless steel weldments,
3. NT TR 548 Improved method for ASTM G48 corrosion testing of welds,
4. Force Technology, Challenges in Pre-Qualification Corrosion Testing of CRAs based on ASTM G48