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I have been working to qualify zinc plating vendors. Part of this involves subjecting the parts to ASTM B117 salt spray. White corrosion after 12 hours or red rust after 48 hours constitutes failure.

Samples from one particular vendor have had a black corrosion product, as seen in the picture. I have done some chemical analysis via EDX and the black areas are about 10 wt.-% Cl. Areas that do not have black corrosion have little if any Cl.

I have done research and I'm not finding a lot about black corrosion on zinc plated products. Does anyone know what could cause this? Would you consider this to be a failure despite the lack of red rust?

Incidentally, the same samples had a streaked appearance in some area as-received (before salt spray). I have also included pictures of that condition at two different magnifications. The streaked areas appeared to be spheroidal nodules of zinc that were above the surface of the "normal areas". I don't know if the two things are related, but they appear to correlate. Samples with more severe streaking had more black corrosion product than samples with little or no streaking.

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Kenneth Kirby
Senior Project Engineer
Snap-on, Inc.
Kenosha WI
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Kenneth, I am a metallurgical/materials engineering consultant but not a plating expert.  The black corrosion product has the appearance of a form of iron chloride.  It would not be desirable (in terms of long-term corrosion resistance) to have iron chloride on the surface of a steel component.  Speculating that contamination of the plating bath (and/or component surfaces) is leading to this condition.  Destructive evaluation, including metallographic cross-sectioning, might be able to provide additional information about the plating layer, underlying substrate, etc.

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Scott Lieberman
Director of Metallurgical and Materials Engineering
EDT Corp.
Brooklyn NY
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Hello Kenneth,

The corrosion products during salt fog testing can easily deciphered. I am assuming this is a steel part. However, before I can comment on this issue, we will need the following information:

1. Type of zinc plating (cyanide, alkaline or acid chloride)
2. Thickness of zinc plating
3. Identification of conversion coating on top of zinc
4. Seal coat

Each of these will influence the appearance of corrosion products and their time to initiate. 

A soft steel part plated with zinc (I am assuming that is what this part is)  may show corrosion products similar to what you are witnessing. Here are my comments:

1. The first corrosion product to form are "black" spots. While these black at low magnification in visible light, they will appear white and fluffy at a higher magnification under visible light. These generally comprise of zinc hydroxides.
2. The "black" spots grow and become areas that appear black in you macrograph. Since zinc hydroxides are quite porous in morphology the absorb some of the salt from the fog and will give you Cl peak in EDX.
3. The white deposits are typically salt deposit contaminated with oxides. Some of them will dissolve if washed with hot distilled water, as recommended by ASTM B117 procedure. These deposits do not constitute failure of zinc plating. 
4. Once all zinc is corroded, you may start seeing red tint to corrosion products, followed by rapid appearance of red rust on the surface.
5. You need to establish failure criteria for both zinc and iron corrosion. Typical failure criterion may be black spots larger than 5 mm dia for zinc corrosion and red spots larger than 5 mm dia for base metal corrosion. 
6. The nodular deposits that you are seeing are typical of acid chloride process under normal current density. Any streaking points to inadequate cleaning after plating.

Controlling condition of testing is also important. The "significant" surface must be inclined 30 degrees to the vertical and should be well drained. The salt deposits appear when salt fog solution is allowed to pool and collect on a part surface. This may point to poor test control. 

I hope this helps.

Ratnesh Dwivedi, Ph.D.

President

RKD ENGG, LLC

www.rkdengg.com

Email: contact@rkdengg.com

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

www.rkdengg.com
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Thank you for your input. To clarify, the part is steel, but it is heat treated, not soft. I need to get a little more information to answer all four of your questions and will reply again in the near future.

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Kenneth Kirby
Senior Project Engineer
Snap-on, Inc.
Kenosha WI
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Original Message:
Sent: 01-08-2026 09:40
From: Ratnesh Dwivedi
Subject: Black corrosion product on zinc plated steel

Hello Kenneth,

The corrosion products during salt fog testing can easily deciphered. I am assuming this is a steel part. However, before I can comment on this issue, we will need the following information:

1. Type of zinc plating (cyanide, alkaline or acid chloride)
2. Thickness of zinc plating
3. Identification of conversion coating on top of zinc
4. Seal coat

Each of these will influence the appearance of corrosion products and their time to initiate. 

A soft steel part plated with zinc (I am assuming that is what this part is)  may show corrosion products similar to what you are witnessing. Here are my comments:

1. The first corrosion product to form are "black" spots. While these black at low magnification in visible light, they will appear white and fluffy at a higher magnification under visible light. These generally comprise of zinc hydroxides.
2. The "black" spots grow and become areas that appear black in you macrograph. Since zinc hydroxides are quite porous in morphology the absorb some of the salt from the fog and will give you Cl peak in EDX.
3. The white deposits are typically salt deposit contaminated with oxides. Some of them will dissolve if washed with hot distilled water, as recommended by ASTM B117 procedure. These deposits do not constitute failure of zinc plating. 
4. Once all zinc is corroded, you may start seeing red tint to corrosion products, followed by rapid appearance of red rust on the surface.
5. You need to establish failure criteria for both zinc and iron corrosion. Typical failure criterion may be black spots larger than 5 mm dia for zinc corrosion and red spots larger than 5 mm dia for base metal corrosion. 
6. The nodular deposits that you are seeing are typical of acid chloride process under normal current density. Any streaking points to inadequate cleaning after plating.

Controlling condition of testing is also important. The "significant" surface must be inclined 30 degrees to the vertical and should be well drained. The salt deposits appear when salt fog solution is allowed to pool and collect on a part surface. This may point to poor test control. 

I hope this helps.

Ratnesh Dwivedi, Ph.D.

President

RKD ENGG, LLC

www.rkdengg.com

Email: contact@rkdengg.com

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

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