Preferential electrochemical etching of the metal grains or grain boundaries 1

[Spectre50]

We are using a cyanide based electrochemical etching process to etch tapered tips on a precious metal alloy wire and have discovered that each material lot behaves differently with the same etch process. These material lots will preferentially etch the grains in one lot and varying levels of the grain boundaries in the next lots. We have etched these material lots in the same batch and have achieved the same grain boundary etch results. I have found out that that the lots we have purchased are from the same pour but were heat treated at different times. We have analyzed the exact hardness of the lots and found that the degree to which the grain boundaries is etched does not follow the hardness of the material. What material properties would cause the preferential etching of the grain boundary to varying levels or the preferential etching of the grains? The EDX that we had performed showed no differences in composition but that testing is qualitative anyway.

 

[Spectre50]

This is the picture of the grain boundary etching.

 

[Maui]

Time, temperature, and etchant concentration are the typical variables that serve to impact how aggressively the material etches. How the clean (or how dirty) the sample surfaces are prior to etching can also play a role here.

Maui

 

[Maui]

You may also have some precipitation along the grain boundaries from the different heat treat lots that could impact the etching characteristics of these samples.

Maui

 

[arunmrao]

Don't forget,the free energy at the grain boundary is always higher than inside the grain.

"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[Spectre50]

Maui-We have maintained the same process for all lots that are etched. We are etching away a large amount of material so we should be free of any surface contamination. Is there a test that could accurately determine the precipitation along the grain boundary?

Arunmrao- We sometimes etch the grains alone and some times the grain boundaries even though the same process is used. Could the grains sometimes have more free energy?

 

[Maui]

Spectre50, there is variation in every process. Steel or precious alloys from the same heat lot typically experience segregation to some extent during solidification. The degree of segregation depends on the chemical composition of the melt (which is tied to the cleanliness of the scrap materials used in its production), it's temperature during teeming, the method of solidification, etc. Your heat treatment of different lots can introduce variation depending on the temperature variation that exists inside the furnaces, the physical size and geometry of the pieces, how they are cooled, etc. Your etching process also can contribute to variation. Are you etching them all for exactly the same amount of time using the same concentration of etchant and the same voltage and current level? Do they all have exactly the same sample preparation prior to etching? Or are several different technicians performing the sample prep? Differences in how technicians perform their jobs can have a pronounced impact on the test results in some cases - I have measured this myself.

Maui

 

[Spectre50]

Maui- In order to eliminate our process from contributing to the outcome I placed all the lots in the same fixture and etched them at the same time. The results were the same as when we etched them lot by lot. The preparatory procedures are the same with each lot. In order to further verify that our process was not contributing to the problem we tried varying all our possible process parameters with no effect on the preferential etching.

Which test would you recommend to use for the grain boundaries?

 

[Maui]

Spectre50, it is good that you have looked at your sample prep process first and determined that it is not apparently contributing to the issue. But you haven't shared with us what the alloy is yet. What is it? Can you provide a material certification that shows the chemical composition? Depending on their size and geometry grain boundary precipitates could be identified through metallography, SEM analysis, TEM, Auger, etc. The best technique depends on what the precipitates are, if in fact there are any at all.

Maui

 

[EdStainless]

Sounds like SEM work is on order.
The variation could also be related to the grain orientation. Some directions will be attacked faster than others. You may need to look at grain size (by metallography) and orientation also (this would be XRD work).

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P.E. Metallurgy, Plymouth Tube

 

[Spectre50]

Maui-I cannot give out the percentages of the metal as it is patented but I can say that there are high concentrations of PD and Ag and low concentrations of Au, Pt and Cu and a minute concentration of Zn. Does this change the type of precipitate and therefore the needed test?

EdStainless- My current plan was metallography and XRD unless Maui states that a different test is required based on the composition.

 

[Maui]

Spectre50, before recommending that you have any additional testing performed I would like to know specifically how the images that you provided in your original posts were generated. Based on their appearance it lookss as though these were taken in an SEM. And it appears as though one image was generated using backscattered electrons (BSE), while the other was produced with secondary electrons. Is this correct? Could this by itself be responsible for the variation you have identified? Has your SEM operator used these two different scanning modes without telling you? Those two different scanning modes will generate very different appearances for the same type of sample surface.

Maui

 

[Spectre50]

Maui- I am investigating your current questions about the SEM images. I wanted to provide a paper that I located today which states that in aluminum the preferential etching of the grain boundaries increases with increasing Zn content. I will be sending out samples for composition testing as well as phase testing.

 

[EdStainless]

You need to look at finding if there are different phases in or along the grain boundaries. Perhaps secondary phases form in this material either from minor composition variations or HT variations.
You might also want to try re-heat treating some deliberately too hot and too cold just to see what variations you can generate.

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P.E. Metallurgy, Plymouth Tube

 

[Spectre50]

Maui- both images were taken with secondary electron imaging.
Edstainless- I will let our lab know.

 

[Maui]

I would recommend starting with metallography and SEM analysis to determine if you have precipitates along the grain boundaries. Ed's suggestion to purposely alter the heat treatment by increasing or decreasing the temperature has merit as well. Time at temperature can be a factor here too. Are all of these specimens being held at temperature for the same actual length of time, or is there variation in this parameter? Your recipe probably dictates a specific length of time at temperature, but is this being followed?

Maui

 

[Maui]

How is the electrical current beng delivered to these very small tapered tips? The manner in which this is being done may also induce some variation in the results.

Maui

 

[Spectre50]

MAUI-The power is being applied to a block of stainless steel. The parts are held against this block with a rubber hinged arm. When we tried each lot with separate fixtures we received the same result as putting all the lots on the same fixture. To the best of our knowledge our heat treatment personnel is following the proper procedures. We have asked many times but the answer has remained the same.

 

[EdStainless]

When it comes to the heat treatment I would ask this way, When was the last QA audit? The last furnace profile? The charts from these runs? Photos showing that the loading was the same? These guys should be falling over themselves to show to you that they did it right.

I would still focus on segregation and secondary phases.

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P.E. Metallurgy, Plymouth Tube

 

[btrueblood]

Not that this may apply to your problem, but we had trouble with a lot of nickel-alloy tubing that was highly sensitized to subsequent brazing (intergranular attack by the braze alloy). We finally determined, after review of processing controls (see Ed's reply) with the various vendors, that a lot of tubing had not been sufficiently cleaned of drawing lubricant prior to an in-process anneal, which created a lot of excess chromium carbides in the material and subsequent loss of corrosion resistance. SEM/EDX at the time was not showing us the carbon content, so we were not learning the whole story just from the micrographs.