WA Branch Report

WA Branch Technical Meeting – 9 March 2020 Localised Corrosion of Additively Manufactured Stainless Steel: Which Evaluation Method Can We Trust? Source: Dr. Mobin Salasi, Curtin Corrosion Centre

Dr Mobin Salasi gained his Bachelor and Masters degrees in Iran, before undertaking research on corrosion issues in the oil industry over four years. He then returned to study, gaining his PhD, on Synergism between abrasion and corrosion, from The University of Western Australia.

At the recent Western Australian branch technical meeting, Dr Mobin presented on the topic, Localised Corrosion of Additively Manufactured Stainless Steel: Which Evaluation Method Can We Trust?

This work was prompted by claims that additively manufactured (AM) 316L stainless steel shows better corrosion performance than the wrought equivalent. This justified a laboratory study, supported by Woodside, to investigate the claims. The AM specimens were prepared at The University of Toulouse.

Initial tests appeared to confirm a promising result, but inconsistencies indicated the need for further investigation. The investigations revealed that behaviour depends on which accelerated evaluation method is used. Dr Mobin explained the reasons for the discrepancies, and his conclusions regarding the claims of superior performance of AM stainless steel.

The corrosion resistance of stainless steels depends on the presence of a passive oxide film on the surface; if this breaks down, corrosion will proceed. The most common initiators of passive film breakdown are pitting corrosion and crevice corrosion. Pitting corrosion is essentially microstructural, while crevice corrosion occurs in regions shielded from the general service environment.

In both cases, the corrosion rate depends on the product of corrosion current and distance of the corroding regions from the outer environment. The sequence of corrosion is deoxygenation, followed by a concentration of metal ions, hydrogen ions and chloride anions to maintain charge balance, leading in turn to de-passivation, and continued propagation of the corrosion remote from the outer environment.

L to R: Dr Paul Huggett, Dr Mobin Salasi

Subsequently, localised corroded areas coalesce, resulting first in crystallographic attach (etching) and finally in uniform (polished) attack.

Rapid evaluation of propensity for localised corrosion is commonly undertaken using several tests. These include cyclic polarisation, potentiodynamic (PD) polarisation, and a combination of PDpotentiostatic-PD testing, with or without ceramic or Teflon crevice formers, and in either standard solutions, or solutions that replicate conditions inside a pit.

Potentiodynamic testing, without a crevice former, indicated that AM 316L can show a much higher breakdown potential than the wrought material, indicating higher resistance to pitting. However, repeated testing showed variable results. Other accelerated tests did not confirm superior resistance, and longer term free potential tests, replicating normal service conditions, led to the conclusion that, “316 is 316, regardless of whether it is additively manufactured or wrought”.

But, why do some tests show a higher breakdown potential? In wrought stainless steel, manganese sulphide inclusions are known to initiate passive film breakdown. However, these inclusions were absent in the AM material. The AM method used

was laser fusion of 30-40 µm powder, by which the localised molten pool was cooled extremely rapidly by the surrounding metal.

Microstructural study revealed siliconmanganese-oxygen inclusions, but these were at the sub-nanometre scale and thus had no effect on crystallographic corrosion attack. This led to the observations of high, though variable, breakdown potential for AM 316L in potentiodynamic tests. Once any random feature or event initiated local de-passivation, the AM alloy showed the same behaviour as wrought specimens.

Dr Mobin’s conclusion is that accelerated testing is not a reliable indicator of improved local corrosion resistance in AM stainless steel. Evaluation must be made over a long enough time to allow for the action of all factors that could initiate depassivation. Long-term immersion remains the most reliable method.

Answering questions, Dr Mobin remarked that while the availability of cheap 3D printers might give the impression that AM is a simple process, dealing with 30-40 µm metal powder is hazardous, and requires clean-room conditions. Nevertheless, it is already widely accepted in aerospace applications and is of increasing interest to the petroleum industry.