To secure the reliability of critical company assets, parts and equipment must be protected from corrosion. During machine operation, oil is circulated or splashed to ensure additives are well dispersed throughout internal system surfaces. The movement of the fluid allows for proper protection, but once equipment is shut down and stored until a later date, standard lubricants often fail to protect unwetted surfaces from corrosion.
Several options are available for protecting equipment during these idle stages. One is to introduce a corrosion inhibitor to the system. Corrosion inhibitors come in many forms with various functions to ensure the equipment is protected. The second is to supplement the system with a mist lubrication or circulating system. Alternatively, mist lubrication systems release a fresh rust-inhibiting lubricant supply to stationary equipment internal surfaces, which ensures a fresh dose of additive is always available. This guide focuses on corrosion inhibitors introduced by the equipment builder and/or users.
Corrosion inhibitors must be able to protect exposed surfaces, and this can be accomplished in many ways. Liquid-phase corrosion inhibitors ensure surfaces covered by the liquid will be protected by the strong additives in the fluid. Vapor-phase protection may be included with the liquid-phase protection or used in dry reservoirs. It works by filling the head space with a vapor that prevents corrosion. Surface coatings protect systems by adhering to the surface. Generally, surface coatings repel water from the surface and include an additive to reduce corrosion at the surface.
When it comes to using corrosion inhibitors to protect surfaces under the level of the liquid, there are many options. The first is a supplementary additive blended with the operating lubricant. These additives boost the corrosion resistance of the lubricant and suppress corrosion risks on remote surfaces where the additive may become depleted over time. The second is a robust corrosion-resistant lubricant designed to fill the system either to normal operating level or 100 percent full. These products provide similar protection as the additive treatments. One risk of introducing an additive to the operating lubricant is incompatibility.
Intermittent operation is common in standby systems that are used only when the main system fails. For these systems, a corrosion inhibitor allowing interment operation is needed. Some of the liquid-based corrosion inhibitors will also function as a lubricant for intermittent operations.
To protect the headspace or gaseous voids in a system, vapor corrosion inhibitors (VCIs) are usually needed. VCIs come in many forms and are often combined with liquid corrosion inhibitors to provide protection above and below the fluid level. VCIs generally need to be contained in sealed systems where the head space can be filled with vapor. Most VCIs are a liquid or powder that will slowly release corrosion-inhibiting vapor into the head space. Fluids that are intended for stored equipment or interment operations will typically contain both liquid-phase protection and VCIs.
When equipment or parts are stored or shipped, it is often necessary to maintain a dry sump. This is where surface coatings become useful. There are three main types of surface coatings: dry, greasy and oily coatings. Generally, a greasy or oily coating is used for bearing storage to ensure its protection. These are often referred to as slushing compounds. They perform multiple tasks, but the two main functions of the coatings are to prevent corrosion at the surface and to prevent moisture and oxygen from reaching the surface.
These coatings can be applied in various manners. They can be brushed on, sprayed on or a part can be dipped into the corrosion inhibitor. They are normally formulated with a solvent that will flash off, leaving behind a more resilient product. Many surface coating can be delivered in aerosol cans.
Unless the corrosion inhibitor is that which is formulated in the lubricant to be used in the systems, there will likely be compatibility issues. Some issues may go unnoticed, while others can cause serious side effects. These could vary from reacting with the lubricant’s additives to clogging filters in the system. It is best practice to ensure the system is properly flushed and cleared of any residual fluids in advance. The article “How to Flush Gearboxes and Bearing Housings” by Mike Johnson is an excellent source of information for flushing.
Assumed compatibility is a common problem. For example, slushing compounds used on new bearings are generally compatible with mineral oils, but are not always compatible with synthetic oils. Always make sure the corrosion inhibitors and lubricating fluid are compatible.
Some corrosion inhibitors can simply be flushed with the operating fluid, while others must be removed with solvents or cleaners to strip the additives from the surfaces.
Besides compatibility issues with the operational lubricant, other side effects should be considered when purchasing a corrosion inhibitor, including its compatibly with materials in the systems. Ferrous, nonferrous, plastic and rubber are all used in machinery today. It is essential to verify that the fluid used to protect a system doesn’t attack it.
Side effects can also be created by the environment. In an article from Material EASE, a side note covered the topic of incompatible preservatives in aircraft hydraulics. The preservatives in the fluids were creating an organic plaque on the filters causing premature clogging. It was determined that the high-temperature operating conditions of the hydraulic system was incompatible with the preservative being used.
Brigitte Battat and Wade Babcock. “Understanding and Reducing the Effects of Contamination on Hydraulic Fluids and Systems.” Material EASE. AMPTIAC 21, Vol. 7 No. 1.