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topic 27702

Silver corrosion by zinc based additives in lubricants (ZDDP)

A discussion started in 2004 but continuing through 2018


Q. I am an Industrial Lubrication Specialist, Currently employed by a petroleum products company in San Diego California, USA. I have encountered a marine application involving an EMD (General Motors Electromotive Division) Engine in an oceangoing tugboat that has Silver plated bearings within the engine. Zinc or the "ZDDP" (zinc dithiophosphate) oil additive is forbidden in this application because the zinc will degrade the silver plated bearing resulting in engine failure. The total amount of Zinc in solution cannot exceed 10 ppm.

Can anyone advise the process with which the zinc degrades the silver plating? And why is the use of "Galvanized" fittings in the pumping system also forbidden?. I realize that "Galvanized" fittings are zinc coated, however how will the zinc get into the lubricant by just passing over the galvanization? Is this an ionic exchange. I need to educate my customers as well as myself.

Thank you in advance.

William S. Howes
Industrial Lubrication Specialist. - Desert Hot Springs, California, USA

January 8, 2013

Q. I am a Forensic Engineer investigating the failure of an EMD engine used for power generation, in which it is possible oil containing ZDDP was used, so I would be interested in any feedback you receive on this.

John Towler
- Calgary, Alberta, Canada

February 24, 2013

A. It sounds like what you're dealing with here is galvanic corrosion of some flavor or it might be a reaction between the sulfur in the thiol groups in the ZDDP as it breaks down in solution (it dissolves nicely in nonpolar solvent).

The telling bit will really be the bearings themselves, if you end up with a silver sulfide coating then it probably came from the ZDDP (unless there is another source of sulfur in the engine system).

The easiest way to see this would be to setup an experiment and see what's what.

I would really love to know the answer to this one as well if you happen to find out, more from general curiosity than anything else.

Marc Banks
- Boone, North Carolina

May 20, 2014

Zinc dithiophosphate

A. ZDDP (see Wikipedia) is oil soluble source of phosphorus and sulphur, it provides AW/EP and antioxidant functions. The Zinc atom in the additive exchanges with the silver in the bearing, giving you a bearing that does not work and an additive that is altered (the additive may actually be better, but that is too complex for this answer). The presence of any zinc in the system can cause a similar problem, partly in fresh oil, but significantly in used oil. The presence of organic acids (more a problem in used oil where oxidation has created weak organic acids) will take the zinc, then exchange it in the bearing in a similar way.

Solution, zinc free system or a better bearing (anti friction bearing would be much more suited to an engine in my opinion; I know BMW played with them, also Ducati used rollers for some bikes).

Tim hutchings
PTLube - Hungerford, Berks, UK

July 24, 2018

Q. Further to the list of comments, are there actually any bearing with silver in them in modern EMDs? I was told the slipper bearings under the pistons in turbo engine used to be silver but are no more. And, that the PGA governors had some silver in them, but no more.

Are there any bearings containing silver that would be affected by zinc containing oils in the parts now supplied for EMD engines?

Tim McKeil
McKeil Marine Limited. Tugs, barges, ships. - Burlington, Ontario, Canada.

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