Penetrants and penetrating oils are fluids of low viscosity that are used for freeing fasteners or other mechanical parts that have rusted. Most penetrating oils have a solvent of low viscosity or a medium that is volatile. The liquid penetrates the pores of the metal and deposits a layer of oil (usually solvent based) that causes any present water to be displaced and provides a protective layer to the metal against moisture. Penetrants and penetrating oils may also function as cleaning agents, or corrosion inhibitors.

The Function of Penetrating Lubricants

The main function that penetrating oils perform is loosening fasteners that have corroded, seized or frozen with the passage of time. The low viscosity of the fluid allows it to seep and penetrate into grooves, crevices, or even pores that are present in the given material’s substrate. Penetrating oils are usually mixtures of a lubricant along with a solvent. The solvent thins the lubricant out, decreasing its viscosity and increasing its mobility.


One of the things that penetrating fluids can do is provide lubrication for a short duration of time. Due to the nature of the fluid, friction is reduced but since the oil contains a volatile solvent, they can get contaminated easily. When they evaporate they leave a residual film. This is why penetrating lubricants have additives mixed to improve the longevity of the liquid.

Cleaning Agent

Penetrating lubricants are extremely effective as cleaning agents. They can be used to remove tar, grease, adhesives and rust. This is because the fluid can be used as a solvent cleaner or degreaser for dissolving contaminants. Once the fluid has mobilized the residue, it can be easily wiped away using a wipe, cloth or light abrasive.

Resistance to Corrosion

There are penetrating oils that are used for preventing corrosion. The majority of penetrating fluids have the ability to disperse water, which results in preventing oxidation. Penetrating fluids displace all the moisture in the area where they are applied. Yet other penetrating fluids leave a filmy layer, which is non-conductive. Some fluids are dispersed using a corrosion inhibitor, which causes a passivation layer to be created that acts as a barrier against corrosion.


Straight Oils

Straight oils are oils that are non emulsifiable. They are used in machining processes in their undiluted form. They are made up of base petroleum or mineral oils and usually contain polar additives like vegetable oils, fats, esters, or extreme pressure additives like sulfur, chlorine, and phosphorus.

Emulsion Fluids / Water soluble fluids

Penetrating fluids which are emulsions or water soluble have relatively higher dilution and include a class of fluids which is designated high water content fluids (‘HWCF’).  Fluids which contain soluble oil create an emulsion when they are combined with water. These oils are diluted for use and their levels of concentration go up till 10%.

Semi-synthetic or synthetic fluids

Penetrating oils that are synthetic or semi-synthetic are usually made from synthetic polymers like polyglycol, diesters, esters, silicone, chlorofluorocarbons (CFCs), and a mixture of water and synthetic fluid. Synthetic fluids are known for extremely high resistance to fire and is highly costly.


Extra pressure (EP) additives

The purpose of adding extra pressure (EP) additives is to improve resistance to wear and tear and for the lubrication of metallic surfaces. Extra pressure (EP) additives include chemically reactive agents like phosphorous, sulfur, or chlorinated compounds. These compounds are chemically unstable. Hence, given a high stress environment, they react with a metal that is present to form a coating. The coating is a thin film, made up of a mixture of metal chlorides, sulfides or phosphides that avert any kind of adhesion by completely eradicating contact between metals.

Corrosion inhibitors

The purpose of adding corrosion inhibitors is to eliminate or minimize corrosion. Ideally, an inhibiting agent produces the necessary effect when it is there in lower concentrations. Inhibitor additives usually stick to the surface as a result of electrostatic forces of attraction between the treated surface and molecules or, reacting with the surface to create a passivation coating.

Micro dispersants

The purpose of micro dispersants is to completely envelop liquid and solid particles like graphite, boron nitride, molybdenum disulfide, or polytetrafluoroethylene (PTFE). In the majority of cases, the lubrication is provided by the dispersants once the carrier or solvent has evaporated.

Selecting a penetrant / penetrating lubricant

The selection of a penetrant and penetrating lubricant requires analyzing product features and specifications.

Flash point

The flash point of a penetrant or penetrating lubricant is the lowest temperature at which the fluid evaporates to form a mixture in the air which is ignitable, near the liquid’s surface. Usually, penetrating oils are volatile and have flash point which is low. This makes the flash point an important feature to be considered, especially in electric applications, where it is possible for arcing to occur, or applications involving high temperature. Synthetic fluids and HWCFs have higher flashpoints compared to straight oils.

Operating temperature

Operating temperature is the range of temperatures in which the given fluid exhibits its effects.

Dielectric Strength

The dielectric strength of a penetrating fluid represents the extent to which it can resist the passage of electric current before it decomposes. Fluids with high dielectric strength are used in situations where there is a passage of high electric currents such as telecommunications or power transmission.


Water displacement

Penetrating lubricants with water displacement features can remove water from a surface. Fluids that have lower surface energy relative to water will move beneath the moisture or water on the treated surface.

Non-foaming/Low Foam

The application of penetrating lubricants that are non-foaming or low-foam results in either no foam or very little foam. Additives are used to give these characteristics to the liquid. Foaming can prove to be extremely harmful in cases where the fluid is being used as a coolant or in a grinding application.

The WD-40 Specialist Penetrant Spray is an excellent all-round penetrating lubricant as well and contains compounds designed specifically for loosening up rusted metal.