This is taken from the Motor Oil bible. It talks about sacrificial additives that deal with corrosion and acids.
"RUST AND CORROSION INHIBITORS
Rust inhibitors are special compounds blended into a motor oil that, in addition to the motor oil itself, attach themselves to internal components and prevent the formation of rust by forming a barrier that prevents water from contacting the metal surface. This additive is extremely tenacious and once it attaches itself to the component it will remain there in order to do its job, especially during engine shutdown. This additive is sacrificial in nature and does deplete with time in service.
The only way to determine if these additives are still present in sufficient quantity to effectively prevent rust is to perform oil analysis testing or use the specific brand/type of motor oil according to the oil manufacturers specified change intervals.
There are a two brands of premium quality synthetic motor oils on the market that are designed and engineered for extended drain intervals of 25,000 miles/1-year and one brand engineered for up to 35,000 miles/1-year in which, when used according to the oil manufacturer's recommendations, will provide exceptional rust and corrosion prevention for the entire mileage/time interval.
In order to use any motor oil past the oil manufacturer's recommendations oil analysis testing must be used. Oil analysis testing is covered in another section of this website.
Corrosion inhibitors are blended into motor oil and serve the functions of preventing corrosion of internal engine bearings made from a mix of copper, lead, aluminum and tine. The acids formed in the oil are extremely corrosive and are a result of the combustion process of gasoline and diesels fuels as well as the additives that were blended in with the fuel itself. These by-products of combustion are deposited on the cylinder wall portions that are exposed to the combustion flame front above the top of the piston and then carried into other components by the oil.
Direct blow-by is also a cause of acidic contaminants in the oil. The amount of blow-by in a particular engine is dependent on many factors, with the primary one being the effectiveness of the seal between the piston rings to the cylinder. The acids formed as a result of this will corrode internal parts such as bearings, pistons/cylinders/rings, rockers, camshafts, valves, timing gear teeth and other ferrous and non-ferrous components within the engine.
There are two primary types of corrosion inhibitor chemicals and functions: one is for the additive in the oil to chemically bond to the internal parts and provide a sacrificial barrier and the other is to actually neutralize the acids so that the corrosive potency is reduced to a level where it cannot do any internal damage. This additive depletes with time in service.
Common additives for these purposes include Zinc, Phosphorus and Zinc Diethyl Dithiophosphate (ZDDP), Calcium and Barium. Barium Sulfonates and Calcium Phenates are common chemicals that are engineered with a high amount of the alkali metals Barium and Calcium in order to provide adequate neutralizatization capability specifically due to the alkalinity of these metals. Sulfur content in both gasoline and especially diesel fuel are one of the primary causes of acids in a motor oil.
OXIDATION INHIBITORS
Oxidation is the result of oxygen mixing with oil at engine operating temperatures. It is not so much the amount of oxygen absorbed by the oil that is important, but the amount of oxidation products formed. Oxidation causes an increase in oil viscosity as well as the formation of acids, resins, lacquers and varnish on internal parts, and especially on pistons and piston rings. More severe oxidation occurs as engine operating temperatures increase.
The effect of varnish, resins and lacquers on pistons and piston rings can cause a decrease in the amount of heat transfer between the piston and cylinder as well as stuck piston rings, leading to severe engine damage over a period of time. If the temperatures continue to increase to extremes then these deposits will continue to oxidize into very hard carbon type materials. When this hard carbon material meets with combustion residues and water, sludge is formed. Sludge can do further damage such as plug and block critical oil passageways and oil pump pick-up screens.
In order to decrease the effects of oxidation, oxidation inhibitors are used which disrupt the chemical reaction that is responsible for the formation of the oxidation as well as chemicals that actually decompose the oxidation products already formed. The lacquers, resins and varnish are not only formed at high temperatures by the oil, but also a low to medium operating temperatures by the fuel combustion process. There are numerous very complex chemicals that are used as oxidation inhibitors and it is not my intention to go into the detail of what these chemicals actually are. "
