Laboratory Testing & General Mineral Processing Engineering

Laboratory Testing & General Mineral Processing Engineering

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Steel Corrosion (2 replies and 1 comment)

A
Ashley5584
6 years ago
Ashley5584 6 years ago

Hello,

I work for a tier-1/2 metal stamping plant for the automotive industry. I have a couple of questions regarding corrosion.

We are supplied M10 nuts with Nycote covered threads that are projection welded onto our stamped steel brackets. We've recently received a complaint from our customer because they found rust and corrosion in the threaded area.

Below is some need-to-know information regarding the nuts and processes: 

  1. General Tolerance: MBN 10369 (Displayed on the print: MBN 10369-B-M10-ST-NYC-FE/ZN4)
  2. Per MBN 10369: Surface protection: standard version Fe/Zn4 in accordance with DIN EN ISO 4042, acid zinc coated without passivation.
  3. Nut supplier process: Washing: oil residue; Washing: swarfs; Threading Operation: axial run-out; Threading Operation: thread; Washing: oil residue; Washing: swarfs; Heat treatment-external: hardness; Surface treatment-external: acid zinc; Surface treatment-external: thread coating *Nycote*; Inspection: functional testing; Final Inspection: dimensional testing.
  4. Our Process: Stamp brackets, projection weld nuts onto bracket, External ZnNi plating.
  5. Plater Process: Soak Cleaner bath: cleaner concentration 7.2 mLs min (Techmatic); Electroclean; Water rinse; Acid dip: Hydrochloric 20-40%; Water rinse; ZnNi Plating; Water rinse; Sulfamic acid: pH 1.0-1.5; Chromate; Water rinse; Drying: 120° min. (details attached)

It is my understanding that passivation is the main specification for oxidation inhibition. Cutting, grinding, finishing, and welding can alter the surface, and free iron is released resulting in a lower chrome-to-iron ratio, which can initiate corrosion. Without passivation, the steel is subject to moisture and it's not possible to prevent oxidation. And this was my argument to the Customer since they specify this standard, and that standard specified without passivation.

So here are my questions: 

  1. Is my argument to the customer accurate or could something in the process possibly be causing this?
  2. What is the blue stuff on the threads in the attached picture? It kind of looks like the blue corrosion that builds up on battery terminals and it has sort of a semi-dried puddy-like texture. 

Thank you in advance for your expertise! I was never any good with Chemistry 🙂

*FYI: We're located in East-Central Alabama where humidity thrives...

ZnNi-Plating-Process.pdf
corrosion
https://www.911metallurgist.com/wp-content/uploads/2017/11/Corrosion.png
J
Jorge
6 years ago
Jorge 6 years ago
Like

Based on the information and pictures, I think the blue deposit is nickel chloride, probably you have problems with the water quality and rinse process. Try to review it. If the rinse process does not remove any remaining part of hydrochloric  acid, there will be some problems later.

A
Ashley5584
6 years ago

Thank you! And are you referring to the rinse process from the nut supplier or the assembled part plater that we send the parts to?

J
Jorge
6 years ago
Jorge 6 years ago
Like

Considering the fact that you use hydrochloric acid in the Plater Process, try to review the rinse process associated to it. If the rinse process in the Plater Process is ok, you should get some information on the oil removal operation. In this way to can cover the whole process.

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