In 1990, I encountered a problem using an industrial-equipment manufacturer's supplied gear lube. ..It didn't take long for the brass/bronze components to be attacked. ...The company suggested a different synthetic gear lube, but that was no better. ....I did some research at that time and found that some synthetic gear lubes are marked as being safe for use with "yellow" metals, while many are not safe.
The "GL" ratings do not really indicate which lubes are safe. ...All GL-1 are safe, but they are dino based, not synthetics. ...Some GL-4 and GL-5 lubes are "safe" while others are not.
Here's something from the 'net:
"Yellow metals, as the name suggests, are yellowish in color. They are alloys that contain copper. A standard definition would be a type of brass having about 60 percent copper and 40 percent zinc. Bronze is another type of yellow metal. These metals have been used for centuries to form gears and other components of simple machines.
The problem is that many extreme pressure oil additives contain a form of Sulphur, which will react with the yellow metal.
Two different types of sulfur may be used within these additives. The first type is active sulfur. Sulfur in its active state readily reacts with metal surfaces to form a ductile metal soap that is sacrificial and allows opposing surfaces to contact one another with minimal damage. Active sulfur is chemically aggressive, and with yellow metals being softer than steel, they can begin to pit and form spalls due to this chemical attack.
The second type of sulfur used within extreme pressure oil additives is inactive sulfur. It is less likely to bond to surfaces and react chemically.
The best way to judge whether the form of sulphur in the extreme pressure oil additives is likely to cause damage, is to look at the results of the copper strip corrosion test (ASTM D130). In this test, a strip of copper is immersed in the fluid to be tested at 40 degrees C and again at 100 degrees C. The strip is removed after each test and checked for staining of the copper. The results range from, very little to no staining, all the way to very dark stains. If the results are in the darkish area, then the yellow metals in your worm gearboxes could be at risk of chemical attack.
Rising temperatures can increase the rate at which this reaction takes place. This is explained by the Arrhenius rate rule, which states that the rate of a chemical reaction doubles for every increase of 10 degrees C (18 degrees F) in oil operation temperature.
Active sulfur in some extreme pressure oil additives react with the copper within the brass or bronze. Sulfur, when in contact with copper along with the presence of heat, forms copper sulfide.
This simple chemical reaction can have devastating repercussions on the reliability of machines. In extreme pressure situations, copper disulfide can be formed. Both of these crystalline forms of copper are very hard and can cause abrasive damage to soft machine surfaces."
