Zinc has been a key additive in hydraulic oils for decades. Its primary purpose as an additive is as an anti-wear agent.  The zinc forms a sacrificial layer in the form of Zinc Dialkydthiophosphate (ZDDP). ZDDP protects pumps and other parts of the system from wear damage during boundary lubrication instances.  

The advantage to hydraulic oils with ZDDP is that it will also work as an antioxidant - extending the useable life of the oil.  This can help keep the cost of hydraulic fluids down as it can reduce both the number of different additives needed because it is performing two jobs at once.  

So why do we formulate zinc-free hydraulic oils then?  

ZDDP has a couple of disadvantages as well. Zinc-based additives can be corrosive to certain metals (such as yellow metals). Zinc will chemically attack the metals surface. When a hydraulic system contains these metals the OEMs will require a zinc-free hydraulic oil to ensure the metallurgy of the components are not affected.  

The other main disadvantage to zinc containing fluids is that they have been proven to be aquatically toxic.  When working around bodies of water, such as lakes or rivers, a zinc free hydraulic oil is often used to lessen the risks of any environmental impacts that might occur from a hydraulic leak. 

Can I mix zinc and zinc-free hydraulic oils?  

While it’s never a recommendation to mix different lubricant products, it can be especially detrimental to mix zinc and zinc-free hydraulic oils.  They are formulated with different chemistry and additive packs and those additives don’t always play well together. You can have problems affecting a whole range of important hydraulic fluid factors such as air release time, water separation, foaming tendency, filterability, and even seal compatibility.  

If you need to switch between a zinc and zinc-free fluid you should always do a full drain and flush of the system to ensure that the system is fresh and no mixing of fluids has occurred.