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White spots left by mineral oil on galvanized steel

2007

My company produces pallet rackings. Our profiles (beams and uprights) are produced in-house from galvanized steel coils. I am a civil engineer, so please forgive me if my questions seem too vague or trivial to you.
During the last year, it happened for a couple of times that the lubricant we use in our production lines did not wash away and left white spots on the finished products.
A diffractometry shows higher values of C and O corresponding to the spots. Most likely the spots are given by the solidification of one of the components of the lubricant. The lubricant is mixed with potable water.
My questions are:
1. Did anyone have a similar problem?
2. What are, if existing, the possible reactions involving Zn producing white solid matter? How can I test if this is the case?
3. What are, if existing, the possible reactions involving Cr(III) producing white solid matter? How can I test if this is the case?
I know these are very vague questions, my hope is that someone could at least give me some advice about how to circumstantiate them (as far as I can).
Thank you.

Arturo di Gioia
- Rovereto, ITALY

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First of two simultaneous responses -- 2007

Sir:
What is the pH of your water based lubricant?
Does your lubricant contain Cr+3?
Regards,

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA

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Second of two simultaneous responses -- 2007

White staining of zinc and zinc coatings is due to water remaining on the parts. Worse if the water contains chloride. Sometimes called white rust or storage stain; it frequently occurs when wet parts are stored:

Zn + 2 H2O = Zn(OH)2 + H2 (gas)

Zn(OH)2 is a non-protective white solid. In the presence of good air flow and drying cycles, it gradually converts to a protective layer of basic zinc carbonate monohydrate:

Zn(OH)2 + CO2, in presence of moisture, = ZnCO3·2Zn(OH)2·H2O

This latter product (also whitish, CAS No. 12539-71-8) is much more protective than is zinc hydroxide. Zinc parts should be stored with good air circulation to allow CO2 from air to enter any trapped water or condensation.

Cr(III) forms greenish hydroxide and oxide. Cr(VI) was commonly used in the past to protect zinc from corrosion; the less hazardous Cr(III) is less effective. Perhaps use a lubricant with better corrosion inhibitors, or perform more thorough cleaning and drying before storage.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.

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2007

Dear Thomas and Ken,
thank you for your quick reply.
I'll try to answer your question as thoroughly as I can, please consider that at this moment I have only second hand informations. I'm setting up a series of tests to try to reproduce the problem.

@Thomas:
The mineral oil is basic (pH 9.6 - 10.1 at 3% distilled water). I have four pH measurements, probably made too late with respect to the moment the problem showed up. Three of them gave pretty neutral pH (6.75, 6.89, and 7.14), only one is basic (8.60). The measurement taken from the circuit serving the line which had problems is one of the neutral ones. Anyway, during the next week, while trying to reproduce the problem, I will record a time-history of pH.
As far as I know, there's no Cr+3 in the lubricant, I was thinking to the chemical passivation used by our suppliers, which are switching to Cr+6-free processes.

@Ken:
Can white rust form in a few seconds?
Let me tell you that I've been reported that the problem showed up one day with a few coils from the same source. It appeared both wetting them with the emulsion and with water. Some days later, the same coil had no problem at all. I do not have any evidence about it, only word of mouth (it happened before my arrival and none kept the samples).
White rust appears when the zinc coating is young and has not yet oxidized on the surface, so maybe when the coil has been first used the pure zinc on the surface, maybe not sufficiently protected by the passivation layer, reacted with water, while 5 days later nothing happened because meanwhile the zinc oxides layer fully developed. Could it be the case?
Summary:
- spots appearing under water and emulsion drops in few seconds, before evaporation;
- young coil (used two days after arrival, so possibly very few days after production) showed the problem;
- same coil, 5 days after, did not show the problem.

Obviously if the white spots are zinc carbonate monohydrate, trying to reproduce the problem on the same year's old sample will be a waste of time.
Is there a lab test I could do or ask for in order to identify white rust?

Thanks again for your help.

Arturo di Gioia
- Rovereto, ITALY

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2007

Sir:
My experience (perhaps 20 cases) is with hex chrome, a pH of 4.2, and special additives which work very nicely. I have no experience with trivalent chrome but the fact that a low pH is usually used makes me think it can work.
I have only two experiences with high pH quenches. In one case it was a dulling solution and it did dull, but also caused a white coating on the zinc coating. The other experience also produced a white coating on the zinc. In both cases the white coating was not expected or desired. Likely I cannot help you additionally.
Regards,

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA