ASM Online Member Community

Greetings fellow ASM members,

I am investigating a blister-like defect on a car door, specifically near the weather strip mounting area. The paint and coating sequence on the steel is as follows: pretreatment layer, CED layer, primer layer, base coat layer, and clear coat layer.

After extracting a microsection and examining it under a microscope, the micrograph (second image) suggests corrosion initiating from the base material, as evidenced by the third, zoomed-in micrograph of the second image. It appears there may have been rust on the steel that was not adequately treated before the subsequent layering processes were applied.

My concern is that the pretreatment layer next to the corroded area may not be adhering properly, possibly leading to an increase in corrosion products under the CED or pretreatment layer over time. This could cause the layer to lift from the base layer, as shown in the micrograph.

With this context, I have two questions:

1. Is it possible for corrosion to proceed between protective layers and the initially microscale corroded steel from the base material itself?
2. Could corrosion advance due to moisture seeping through the paint layers to the base material, even with the presence of protective coatings?

Any insights or experiences with similar defects would be immensely valuable

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Vikram Tomar
material engineer 2
atotech
Meerut
07017475745
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Vikram,

The fact that you have steel in the bubbled area is a bit of a concern. I assume that it ended up there during metallographic preparation. Before closing out the investigation, I would recommend preparing another sample after consulting with an experienced metallographer. Generally one might section a distance away from the region of interest, then mount in epoxy and grind into the ROI using 180, 320, 600, 800 (or 1200) grit SiC papers, then polish with a 3 micron diamond film, to keep things flat. This might give you a better look at what is REALLY under the bubble... and better direct you to a solution. 

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David Coulston
Niles MI
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Vikram -

I have to agree with David. Your cross-section with the 50um scalebar
shows unidirectional gouges/scratches greater than 1um wide. You need
significantly finer polish with the removal of polishing debris to examine
the bubbled region.

regards,
- Jim


Original Message:
Sent: 4/29/2024 3:27:00 PM
From: David Coulston
Subject: RE: Investigation on Progression of Corrosion Under Protective Layers on Car Door

Vikram,

The fact that you have steel in the bubbled area is a bit of a concern. I assume that it ended up there during metallographic preparation. Before closing out the investigation, I would recommend preparing another sample after consulting with an experienced metallographer. Generally one might section a distance away from the region of interest, then mount in epoxy and grind into the ROI using 180, 320, 600, 800 (or 1200) grit SiC papers, then polish with a 3 micron diamond film, to keep things flat. This might give you a better look at what is REALLY under the bubble... and better direct you to a solution. 

------------------------------
David Coulston
Niles MI
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Is it also possible that the steel under the bubble is a contaminant that was on the panel before coating (metal shaving from cutting perhaps)? I agree that your suggested method would help prove that one way or the other.

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Kenneth Kirby
Senior Project Engineer
Snap-on, Inc.
Kenosha WI
(262) 748-3836
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Original Message:
Sent: 04-29-2024 15:27
From: David Coulston
Subject: Investigation on Progression of Corrosion Under Protective Layers on Car Door

Vikram,

The fact that you have steel in the bubbled area is a bit of a concern. I assume that it ended up there during metallographic preparation. Before closing out the investigation, I would recommend preparing another sample after consulting with an experienced metallographer. Generally one might section a distance away from the region of interest, then mount in epoxy and grind into the ROI using 180, 320, 600, 800 (or 1200) grit SiC papers, then polish with a 3 micron diamond film, to keep things flat. This might give you a better look at what is REALLY under the bubble... and better direct you to a solution. 

------------------------------
David Coulston
Niles MI
------------------------------

Hello Vikram,

I have seen this type of flaw before in worked (stamped) panels. It may be resulting from inadequate cleaning after the forming process. The forming process generally creates a lot of fine debris that may get incorporated on the part surface. Most of this debris is expected to be removed during post forming cleaning process such as washing, alkaline rinse, acid pickling etc. In some cases, debris may escape cleaning. 

Thanks

------------------------------
[Ratnesh] [Dwivedi]
[President]
[RKD ENGG, LLC]

www.rkdengg.com
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I wanted to extend my sincere thanks for your detailed and helpful response to my question.

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Vikram Tomar
material engineer 2
atotech
Meerut
07017475745
------------------------------

Original Message:
Sent: 04-30-2024 08:15
From: Ratnesh Dwivedi
Subject: Investigation on Progression of Corrosion Under Protective Layers on Car Door

Hello Vikram,

I have seen this type of flaw before in worked (stamped) panels. It may be resulting from inadequate cleaning after the forming process. The forming process generally creates a lot of fine debris that may get incorporated on the part surface. Most of this debris is expected to be removed during post forming cleaning process such as washing, alkaline rinse, acid pickling etc. In some cases, debris may escape cleaning. 

Thanks

------------------------------
[Ratnesh] [Dwivedi]
[President]
[RKD ENGG, LLC]

www.rkdengg.com
------------------------------