Corrosion And Metalworking Fluids

By definition, corrosion is the partial or complete wearing away, dissolving or softening of any metal substance by chemical action. The term 'rust' is often misapplied or misused, as it applies only to iron and steel. The term 'corrosion,' however, is all inclusive, in that it not only applies to ferrous metals but also to non-ferrous metals. The following discusses the causes of corrosion and possible corrective actions.

Article From: 5/15/2003 Production Machining,

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Aluminum Corrosion Samples

Corrosion on metal comes in many forms. Rust, as seen on these cast iron slugs (inset), is common on ferrous materials. Non-ferrous metals too are subject to corrosion as seen by the different staining on these aluminum strips.

US Water Hardness Map

Water hardness affects cutting fluid balance.Different geographic regions have different degrees of waterhardness. Compensating for water hardness will optimize fluid performance.

Chart - Metalworking Fluid Concentration

Every metalworking fluid formula has a "sweet spot" where the concentration should be maintained. Checking the concentration frequently will prevent performance and environmental problems.

Heat removal is one of the most important functions of a metalworking fluid. Effective heat removal yields good tool life and dimensional accuracy of parts. Water has greater capacity for removing heat than oil; however, water alone in contact with freshly machined metal leads to corrosion. Thus, corrosion is a problem faced by every user—and manufacturer—of water diluted metalworking fluids. However, corrosion can occur even with dry cutting and is not simply due to the use of water-based fluids.

Seasonal Corrosion

Corrosion can occur at any time during the year, but normally it occurs more often during July, August and September when temperatures and the relative humidity are high. This applies to the Eastern and Midwestern areas of the United States. Corrosion is not usually as much of a problem in arid states, such as Colorado, New Mexico, Arizona, Utah and California, because the relative humidity is usually low.

As temperature increases, the rate of all chemical reactions also increases. This includes corrosion. High temperature in the presence of moisture and oxygen in the atmosphere is the reason corrosion happens more in the summertime.

Moisture condenses on a part and acts as an electrolyte to form a galvanic cell. If the concentration of the fluid, which provided rust protection during the fall and winter months, does not provide protection when the humidity climbs, an adjustment in the fluid concentration is necessary. If the concentration of 1:30 (3.3 percent) was adequate during fall and winter, then the concentration may need to be increased to 1:25 (4 percent), or to the point where rust is no longer seen. If a fluid user objects to increasing the concentration of his central system mix for reasons such as foam and potential skin problems, it may be necessary to increase rust protection with the use of additives. The additives used depend upon the type of metal(s) involved, the user’s chemical restrictions, additive availability and the fluid used.

pH

The pH of a metalworking fluid is a factor in controlling corrosion. A high pH, greater than 9, will protect ferrous metals but will adversely affect the corrosion control of non-ferrous metals, such as aluminum, brass and bronze.

When the pH is low in an individual machine, the easiest solution to the problem is to dump, clean and recharge with a new mix of the fluid product at the recommended concentration. If treating a central system mix, which is being used on ferrous metals, adjust the pH to between 8.8 and 9.2 with the proper additives. Excessively high mix pHs are usually a sign of contamination, and the mix should be dumped and recharged.

If non-ferrous metals are being machined, and staining or pitting is a problem, check the product information to determine if the product was designed for use with non-ferrous metals.

Water

Chemicals in the water used to make up and maintain the mixes can increase the rate of corrosion. All water contains ions, some of which are aggressive and can cause corrosion of most metals.

Waters that contain more than 100 ppm chloride, more than 100 ppm sulfate, or 50 ppm nitrate are considered aggressive waters. Chlorides, sulfates and nitrates cause corrosion by breaking down the protective barriers on the surface of the metal, opening the way to corrosion. Continuous addition of water increases the sulfate, chloride, and nitrate content of the water in a central system, thus making it more aggressive the longer it is used.

If aggressive water is suspected, take a sample of the water and have it completely analyzed. When a central system fluid is suspected to cause corrosion, submit a sample of the mix for ion determination. If the chloride, sulfate or nitrate content is higher than the acceptable limits, the user may have to change sources of water. It may become necessary, for example, to change to a blend of deionized or distilled water with the regular water. A change to another metalworking fluid with superior corrosion control may also be an answer.

Many desirable properties of metalworking fluids literally can be destroyed by chemical reaction with dissolved water solids. The most familiar example of this is the effect of “water hardness,” which is essentially the calcium and magnesium content of the water.

The divalent ions react with soaps, wetting agents and emulsifiers to form compounds with limited solubility. The formation of these insolubles depletes rust inhibitors, which results in rusty parts and machines. Hard water is any water containing greater than 250 ppm calcium carbonate or 15 ‘grains’ hardness. The higher the hardness is, the more prone the fluid is to corrosion.

Conductivity is another means of determining the quantity of dissolved ions in the mix. Higher conductance promotes corrosion, mix instability, residue and other problems. Conductivities of greater than 4 MilliSiemens per cm are considered high.

Bacteria

High bacteria counts in the fluid mix can lead to corrosion. Bacteria consume metalworking fluid components, and the by-products of this activity can lower the mix pH. The metabolic products include mild organic acids that lower the pH of the fluid and, therefore, the corrosion resistance of the fluid is compromised. Also, if left unchecked, bacteria can split emulsions.

If an individual machine’s mix has a high bacteria count, the easiest solution to the problem is to dump the old fluid, clean the sump with a good machine cleaner, rinse with fresh water, and then recharge with a new fluid mix at the recommended concentration. If it is impractical to dump the charge, treat with an appropriate microbicide. In a central system, use preventive measures including the correct use of microbicides to prevent and control bacterial growth.

Lean Concentration
Ingredients in a metalworking fluid are designed to be effective within a specified dilution range. If the fluid becomes leaner than the specified range, the leaned out ingredients may not be able to perform their designed jobs. This also applies to the rust inhibitors, which may not be able to protect the newly ground or machined parts from corrosion.

Check fluid concentration frequently and routinely. Titration methods and refractometers are the best methods of concentration determination. Add concentrate to obtain the recommended dilution. In individual machines, do not attempt to balance the fluid with the use of additives. Dump and recharge.

Extended In-Process Time

Normal in-process rust protection, provided the metalworking fluid product is used at the recommended concentration, is up to 72 hours. Most fluid products provide in-process rust control for a variety of metals from high alloy steels to cast iron. At the proper concentration, it is reasonable to expect 48 hours protection for cast iron and 72 hours for high alloy steels. If corrosion protection beyond these time limits is required, then the use of a longer term rust preventive is recommended.

Bimetallic Corrosion

Bimetallic corrosion is the corrosion of two different metals in contact with each other and occurs when a transfer of electrons from one metal to another takes place with the cutting fluid acting as a conductor. The solving of bimetallic corrosion problems is a matter of getting a nonconductive material between the two different metals. This may require switching to a fluid that has better bimetallic corrosion prevention properties. In general, this would be a fluid containing a high oil content, such as a premium soluble oil. Waterproof greases or a thin plastic sheet can also be used between the contacting surfaces.

Broken Emulsion

Rust can happen if an emulsion breaks, in which case the instability of the emulsion will make the rust inhibitors ineffective. If the emulsion appears watery and the sample shows stratification, the emulsion may be unstable or broken. To solve this problem in individual machines, the best solution would be to dump, clean the machine, rinse with fresh water, and recharge with a metalworking fluid mix at the recommended concentration.

Broken and unstable emulsions are often caused by hard excessive bacterial contamination. It is important to determine the cause of the broken emulsion before taking corrective action. When dealing with a central system, it may be possible to use an additive that is an emulsifier to re-emulsify the product. If time permits, send samples to the fluid supplier for tests. If time does not permit, then run jar tests on site to determine what additives to use and the proposed dilution ratio.

Process Procedures

Parts sometimes corrode when stacked one on top of the other or when contacting each other in tote bins. This is caused by the fluid acting as an electrolyte and forming a galvanic cell between the two parts. This happens very readily with cast iron. In fact, with stacked cast iron (depending upon the quality iron) it is sometimes difficult to control rust for more than several hours.

Placing a fiber spacer between the stacked layers of parts can eliminate this condition. Do not use cardboard as the spacer because of its high content of sulfur. If using paper or similar materials, be sure they are the vapor phase inhibiting (VPI) type, or they could promote rust. Enclosed tote bins retain moisture and promote corrosion. Use a wire basket for parts or drill holes in the bottom of the tote bins to drain the fluid. If possible, use plastic tote pans or plastic covered wire baskets.

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