Please see the end for a “good-faith” disclaimer.
Resistance to chemical attack and galvanic corrosion are big factors in many designs, especially at elevated temperatures. Even at ambient temperatures corrosion can be an issue: the lowly chain-link fence has its steel protected by a zinc coating which, over time, corrodes instead of the steel – but given enough time, that protection disappears and then the steel itself starts to rust.
In this essay I will outline three examples I have seen where chemicals attacked solid materials, and for each highlight some lessons for people to consider – whether in a design or failure-analysis mode.
Plastics and Acids
At one point in my career I was asked to do some thermal stress analysis because of a series of returned field failures. This was a plastic container made of a relatively new material chemistry whose operation cycle involved being immersed in hot water to heat the contents, and then dunked in cold water to shock-cool them. Over time, cracks would develop in the corners of this injection-molded container. In some instances, catastrophic failure would occur resulting in ruptures of the vessel during the cold-water dunk. To the best of my knowledge nobody was injured in any such event.
The plastic material was believed to be immune to the mildly-acidic contents, so the prevailing theory was that the thermal shock was creating the cracks. This conclusion was logical considering that these failures occurred during the cold dunk operation. My job was to verify this presumed failure mode.
But my first analyses showed something very different than the observed failures. Both partially-failed returns, as well as analysis of sectioned pieces, showed the cracks initiating on the inside of the vessel’s corners. In contrast, every analysis I did showed that any thermal stress cracks should initiate on the outside, where the outer layers – exposed to cold first from the cold dunk – should contract first resulting in crack-opening tensile stresses on the outside preferentially.
Eventually, more detailed and at-temperature chemical compatibility testing showed that the acids inside were attacking the insides of the corners of the vessels.
Result: The material was pulled from this application.
- Chemical compatibility charts and theoretical assumptions are good guides, but verification testing needs to be done on the actual application with real-world conditions.
- Molded-in stresses at the corners likely contributed to the problem; corners in injection-molded pieces tend to have higher stresses than the plastic material of the part in general.
- The thermal shock cycles, while not the root cause, doubtless did contribute. Rapid thermal shocks should be avoided where possible, or included in any factor-of-safety calculations if unavoidable.
Stainless Steel Stress Corrosion
A heavily-welded assembly was found, in the field, to have a pin-hole leak at a weld joint resulting in its hot and pressurized contents leaking out (an obvious safety issue; as above, to the best of my knowledge nobody was harmed). This piece was removed from service and sent for cleaning prior to repair. The cleaning process involved, among other things, the assembly being repeatedly soaked and washed in an agent recommended by the cleaning facility for this type of application and assembly material. A spot-weld repair was then made over the pin-hole leak.
Not long after being returned to service the assembly, which had multiple independent channels only one of which was to be “live” at any time by design and operating procedure, was found to have channel cross-talk – leakage of the working fluid from one channel to another. It was again pulled from service; I was put in charge of the failure analysis.
Even a visual analysis of the piece showed an obvious crack on an internal surface. Pulling prints of this assembly, plus comparable assemblies, showed several differences – only one of which is specifically relevant. To wit: the design (unique to this one variation) resulted in a thin-wall condition in certain locations. A worse-case tolerance stack up would exacerbate the situation significantly.
Sending the assembly out for metallurgical examination, including with it the process history of the cleaning and spot-weld repair including the cleaning agents used, resulted in several key findings.
- Sectioning of the cracked location and subsequent microscope examination showed clear evidence of stress-corrosion cracking, with cracks fully penetrating from one channel to another. The pictures of the cracked area were “stereotypical” of this failure mode.
- The primary cleaning agent, though recommended for the base material, was discovered in a literature search to create stress-corrosion cracking in this material at high temperatures and applied stresses. The suspicion – believed without explicit proof – was that trace amounts of the cleaning agent remained in the relatively-rough surface finish of the machined surfaces.
- Welded structures in general, especially ones with significant machining and welding such as this one, tend to have high residual stresses which are known to play a large factor in stress-corrosion cracking.
- In operation, tightening of threaded fasteners close to the thin-wall area introduced further stresses in that region from the torques required.
- The design, with its thin-wall condition even at nominal design dimensions, was at the root of the problem.
Result: All assemblies of this design were pulled from service and scrapped. Additionally, this particular design concept was ruled out from consideration in the future.
- Check designs for thin-wall areas.
- High temperatures exacerbate any stress-corrosion situation.
- Stresses, whether residual from processing (e.g., machining or welding) or from other operations (e.g., threaded fastener tightening) can also introduce stresses which increase chemical-cracking susceptibility.
- When using any agent to clean, not only vet it with both the base material supplier and chemical agent manufacturer’s recommendations, but check with a literature search to see if similar situations have been reported as causing failures.
- In parallel with the above, go above-and-beyond when it comes to rinsing, in particular if the surfaces being cleaned have a rough surface finish. (And if there is a thin-wall area and it is visible and/or directly accessible, spot-rinse the area to beat heck.)
- This investigation relied heavily on the expertise of a skilled consultant; in this case, a metallurgist. When specialized expertise is needed, don’t hesitate to ask an expert once you realize you do not have the requisite knowledge.
Galvanic Action Surprise
In one application where a cermet component was used in an oil-based environment, the assembly where these small cermet components were used was initially had a carbon steel base part. For machining convenience and better machining-process yield, a switch was made to a stainless steel base.
Not long after this change a new failure pattern of the cermet pieces was observed in parts returned from the field. Initially thought to be a defect in the cermet piece, both the cermet piece supplier and an independent metallurgist provided links and other information pointing to galvanic corrosion as the likely suspect after being informed of the base material change.
This seemed strange, as galvanic corrosion requires water; this was immersed in oil. However, chemical analysis of the oil showed not only measurable amounts of water, but of materials (salts mostly) in the oil that – when exposed to the entrained water and then dissolved in it – would form a conductive path which is essential for galvanic corrosion to occur.
The switch from carbon steel to stainless moved the situation from one where the carbon steel had been the material degrading across a large surface (and therefore going unnoticed) to one where the much-smaller cermet was preferentially attacked – as stainless steel has a much higher resistance to galvanic corrosion than carbon steel.
Result: The application was switched back to a carbon steel, though a different one than the original for better and easier machining. No further failures of this type were observed.
- Even oil environments can have enough water and trace elements to create a pathway for galvanic action to occur.
- If changing materials for machining considerations, consider a material of a similar type (e.g., in this case merely a different carbon steel), rather than a major change of material type.
- If necessary to change to a material more resistant to galvanic action, keep an eye on materials in the system that have not changed as they may now be attacked by galvanic corrosion when they were not before.
- Most suppliers want the application to succeed; tap them as knowledge experts before making such a change. If they’ve been in the business for a while, odds are they’ve seen similar situations before.
- Again, don’t be afraid to consult with outside subject-matter experts on your dime to be sure there is not a conflict of interest (i.e., the supplier had their own metallurgist; the one we hired was a second opinion without that potential influence of their working for the supplier).
Disclaimer: I was very opaque about details like when and where these occurred, and the specific applications – and deliberately so. I am attempting to tread the fine line separating writing with just enough detail to be educational, and not disclosing anything proprietary (or damaging, though note: there were no dangers from the third example, and in the first two – to the best of my knowledge – nobody was harmed and in all instances situations have been remedied and are no longer a danger going on multiple years now). No company names are mentioned, nor are process parameters, design details, or specific material callouts given except in an “arm waving” mode to set the stage. Should there be a desire to change anything on the part of the companies involved, please contact me directly and make some suggestions as to rewording to remove any information that might currently be creating a material-damage situation – a situation which I have labored in good faith to avoid with my deliberate vagueness.
© 2015, David Hunt, PE