Originally Posted By: Michael_P
Question. If too much zinc can cause corrosion, why is zinc used as sacrificial anodes to protect marine metals such as aluminum and steel? Zinc would be the least noble metal of any metal found in an engine. What about the phosphorous that accompanies zinc, or the dithiophosphate compound? I have seen cam surfaces with rust pocks on them where the mechanic called it zinc poisoning. Furthermore, zinc corrosion seems to be magnified in a marine environment, or at least that is what I have been told. I am by no means a chemist, but know enough to sound dumb. My boat with over 2500 hours on it had zero internal corrosion on a teardown and it has seen 1600PPM zinc oil since day one. I have gone as far as 220 hours on an OCI as well.
Cathodic protection is due to an electro-chemical effect and can hardly be compared to surface interactions in tribology.
Here is a good explanation of cathodic protection and sacrificial anodes:
Cathodic Protection
But you're right Michael, it the concentration of phosphorous and sulfur in the ZDDP molecule.
Zinc Dialkyl Dithiophosphates or ZDDP (or Dithiophosphoric Acid, O,O'-Isobutyl Amyl Ester, Zinc Salt ) is an ester which is used as an Anti-Wear agent and an Anti-Oxidant agent (a multifunctional additive) in oil additives and is composed of the elements of zinc, phosphorous, and sulfur.
When concentrations get above approx. 1200 ppm, and when under high moisture conditions and extremely high temperatures, there is the potential for the ZDDP molecule to split apart (via hydrolosis) and form acids attacking the metals as acids of sulfuric and phosphoric acid.
Marine oils, both gear lubes and PCMO's, as well as classic car and HDEO oils, have a higher level of anti-corrosion agents which tend to combat any acid attack. Most Marine engines also tend to run cooler, only to expel moisture at WOT when hot.