Penetrants and penetrating oils are low viscosity fluids used to free rusted fasteners or other mechanical parts. Most penetrating fluids contain a low viscosity solvent or other volatile vehicle. The fluid penetrates the metal's pores and deposits an oil film (typically solvent based) which displaces water and protects the metal from moisture. Penetrants and penetrating oils may also function as lubricants, cleaning agents, or corrosion inhibitors.
The primary function of penetrating oils is to loosen frozen, seized, or corroded fasteners. Its low viscosity enables the fluid to mobilize and penetrate into crevices, threads, and even the pores of the material's substrate. Most penetrating oils are fluid mixtures with a lubricant and a solvent, which acts to thin out the lubricant and decrease its viscosity.
All penetrating fluids provide temporary lubrication. The fluid decreases friction coefficients, but due to the nature of many of the solvents used they are volatile, being easily contaminated and evaporate leaving only a residual film. For long-term lubrication some penetrating fluids contain additives.
Penetrating oils should not be used as a replacement for industrial lubricants as they do not provide the same fatigue rate.
When used as cleaning agents, penetrating oils are effective at removing rust, adhesives, tars, and greases. The fluid acts as a degreaser or solvent cleaner to dissolve the contaminants. Once the residue is mobilized by the fluid it is easily removed using a tack cloth, wipe, or light abrasive.
Penetrating oils that provide corrosion resistance stop and prevent corrosion. Most penetrating fluids have water dispersing characteristics which stop oxidation on application by displacing moisture. Other fluids will leave a non-conductive film or are dispersed with a corrosion inhibitor that leaves a barrier film or passivation layer that controls or eliminates corrosion.
There are several basic types of penetrants and penetrating oils.
Straight oils are non emulsifiable products that are used in machining operations in an undiluted form. They are composed of base mineral or petroleum oils and often contain polar lubricants such as fats, vegetable oils, esters, or extreme pressure (EP) additives such as chlorine, sulfur, and phosphorus.
Water soluble/Emulsion Fluids
Water soluble and emulsion fluids have a high dilution or very low concentration and include high water content fluids (HWCFs). Soluble oil fluids form an emulsion when mixed with water. These penetrants and penetrating oils are used in a diluted form with concentration levels ranging up to 10%.
Synthetic and semi-synthetic penetrants and penetrating oils are based on synthetic compounds such as silicone, polyglycol, esters, diesters, chlorofluorocarbons (CFCs), and mixtures of synthetic fluids and water. Typically, synthetic fluids have the highest fire resistance and cost.
Extra pressure (EP) additives are added to improve wear resistance and lubricate metal surfaces. They include chemically active agents such as sulfur, phosphorous, or chlorinated compounds. These compounds react with metals in a high stress environment to form a chemical coating. The coating is a thin film of metal sulfides, chlorides, or phosphides that prevent seizure, sticking, or surface adhesion by eliminating metal to metal contact. EP additives excel in high load applications where other lubricants fail.
Corrosion inhibitors are fluid additives used to control the rate or eliminate corrosion. The ideal inhibitor will produce the desired effect when present in low concentrations. Most inhibitors additives either adsorb to the surface due to electrostatic forces between the molecules and treated surface or may react with the treated surface to form a passivation layer.
Micro dispersants are used to envelop solid and liquid particles such as polytetrafluoroethylene (PTFE), graphite, and molybdenum disulfide or boron nitride. In most cases the dispersants provide lubrication after the solvent or other carrier evaporates.
Selecting penetrants and penetrating oils require an analysis of product specifications and features.
Flash point is the lowest temperature at which a fluid's evaporation rate generates sufficient vapors to form an ignitable mixture in air near the surface of the liquid. Most penetrating oils tend to be volatile with a low flash point. In electrical applications, where arcing may occur, or high temperature applications, flash point is an important selection criterion. Straight oils tend to be highly flammable, while HWCFs and synthetic fluids have a much higher flash point.
Operating temperature or use temperature is the temperature range under which the fluid will exhibit a desired effect. The temperature range may be governed by the flash point or may be used to specify penetrating fluids used in refrigeration or cryogenic applications.
Dielectric strength depicts the maximum voltage field the fluid can withstand before an electrical breakdown occurs. Products with a high dielectric strength are used on telecommunication or utility equipment where the fluid may be in contact with an electrical load.
Water displacement lubricants or fluids have the ability to displace water from a surface based on wetting or surface energy characteristics. Fluids with low surface energy or interfacial tension compared to water will flow under the water or moisture on a surface.
Low or non-foaming fluids do not produce foam or produce only small amounts of foam. Non-foaming characteristics are achieved through the use of additives that break out entrained air. Leaks can introduce air into systems for circulating hydraulic fluid, thermal oil, or grinding coolant. Entrained air can cause pump damage due to cavitation. Foaming can also reduce the cooling ability and the bulk modulus (or stiffness) of the fluid.
MIL-PRF-24548 - This specification covers the requirements for a penetrating fluid for direct, brush, or dip application to free corroded and frozen metallic parts without causing damage.
A-A-50493 - This commercial item description (CID) establishes the government acquisition requirements for penetrating oil to be used for freeing of corroded and frozen metallic parts resisting movement, without causing damage.