Corrosion is a chemical reaction that partially or completely peels off, dissolves, or softens the surface of a metal. Rust "is often misused or misunderstood, referring only to iron and steel. "Corrosion" includes not only Ferrous, but also non-ferrous metals. The following content mainly discusses the causes of corrosion and corrective measures.
Removing heat is one of the most important functions of metal working fluids. Effectively removing heat can ensure the good service life of the cutting tool and the geometric accuracy of the workpiece. Compared to oil, water has superior performance in removing heat; But pure water can cause corrosion when it comes into contact with newly processed metal. Therefore, corrosion is a problem that every user and manufacturer of water-based metal working fluids must face. The dry cutting process also faces corrosion issues, which are not only caused by water-based metal working fluids. There are many reasons for metal surface corrosion. Seasonal corrosion Corrosion can occur at any time within a year. Generally speaking, the temperature and relative humidity from July to September are higher, making corrosion more likely to occur. Due to low relative humidity in arid areas, corrosion rarely occurs. Fingerprint corrosion When the workpiece comes into contact with human hands, it is prone to corrosion. The handprints left on the surface of new machine tools and metal workpieces during transportation can cause corrosion. This situation is commonly found in people with acidic skin and workpieces with high surface finish. The use of a fingerprint neutralizer can prevent similar fingerprint corrosion. PH value PH value is an important parameter for controlling corrosion in metal working fluids. A high pH value of more than 9 can protect Ferrous, but is unfavorable to the corrosion protection of nonferrous metals, such as aluminum, brass and bronze. Water hardness can affect the balance of the processing fluid, and water hardness varies in different geographical regions. Adjusting water hardness can optimize the performance of the processing fluid. If the pH value is low under single machine conditions, the simplest solution is to pour and clean, and then add fresh metalworking fluid according to the recommended concentration. If it is a central trough system for processing Ferrous, the pH value can be adjusted to 8.8~9.2 with appropriate additives. If the pH value is particularly high, it is often because the Metal Working Fluid has been contaminated and needs to be poured and replaced with a new one. Dirt recycling Metal particles in metal working fluids are often considered as "dirt" or "debris". If not cleaned in a timely manner, debris can accumulate on the surface of the workpiece and form batteries, and the metal underneath the debris often rusts. Under single machine conditions, it should be drained, cleaned, and rinsed with clean water in a timely manner, and fresh metal working fluid should be added according to the recommended concentration.
Water
Usually, chemicals in water accumulate and increase the corrosion level of the processing fluid. All water contains ions, some of which are corrosive and can cause corrosion of most metals. Water contains over 100 × 106 chloride, over 100 × 106 sulfides, or 50 × 106 nitrate, these ions are considered aggressive. Chlorides, sulfides, and nitrates damage the protective layer on metal surfaces, leading to corrosion. Continuously adding water will increase the chloride, sulfide, and nitrate content of the central tank system. The longer the metal working fluid is used, the higher the corrosiveness of ions.
The formula of each metal working fluid needs to maintain its concentration to achieve its "best point". Regularly testing the concentration of metal working fluid can avoid processing performance and environmental issues.
If the user suspects that the water is corrosive, they can take a sample and determine it through full analysis. When the metal working fluid in the central groove system is suspected of causing corrosion, please take a sample and test the ion content. When the concentration of chlorides, sulfides, and nitrates exceeds the acceptable range, deionized water or distilled water can be used as process water, or metal working fluids with high corrosion resistance can be selected.
Solids dissolved in water can disrupt many of the desired properties of metal working fluids. The most familiar example is "water hardness", which is caused by the dissolution of calcium and magnesium ions in water.
The solubility of compounds formed by the reaction of divalent ions with soaps, wetting agents, and emulsifiers will decrease. This insoluble component depletes the rust inhibitor of machine tools and workpieces. Hard water refers to water content exceeding 250 × 106 calcium carbonate or 15 "German grams" (German hardness standard). The higher the hardness, the more prone it is to corrosion.
Conductivity is another method for detecting dissolved ions in metal working fluids. High conductivity increases corrosion, instability of metal working fluids, residues, and other issues. Exceeding 4MilliSiemens/cm is considered high conductivity.
Bacterium High bacterial content in metal working fluids can lead to corrosion. Bacteria not only deplete the components of the processing fluid, but also lower the pH value of the mixture. The metabolites of bacteria include weak organic acids, which can lower the pH value of the processing fluid and weaken the corrosion resistance of the metal processing fluid; If not tested, bacteria can break the emulsion of the metal working fluid. For a single machine, the metal working fluid contains a large amount of bacteria. The simplest solution is to pour the old working fluid, thoroughly clean the entire system with an efficient machine tool cleaner, rinse with fresh water, and then add fresh metal working fluid according to the recommended concentration. If it is difficult to replace the new solution, appropriate fungicides can also be used. For the central tank system, preventive measures can be taken, including the correct use of fungicides to control bacterial growth.
Low concentration
Within the scope of design, the composition of the metal working fluid will be effective. If the concentration is low, the composition of the metal working fluid cannot achieve the designed concentration effect. Including rust inhibitors, they cannot prevent corrosion of newly processed and ground workpieces.
The concentration of the processing fluid should be regularly or regularly tested. Chemical titration and refractometer are the best methods for detecting concentration, and concentrated solution can be added to achieve the recommended concentration. For single machines, additives should not be used to break the balance, and the liquid should be poured and replaced.
Long process time Generally speaking, the recommended concentration of metal working fluid can provide a maximum of 72 hours of rust prevention time between processes. The rust prevention time between processes in the vast majority of metal working fluids varies with the changes in metal materials (such as high alloy steel, cast iron, etc.). The acceptable rust prevention time for cast iron at an appropriate concentration is 48 hours; High alloy steel takes 72 hours. If the rust prevention requirements exceed these times, it is recommended to use long-term rust prevention additives. Corrosive gas After the metal processing process is completed, the workpiece enters the storage period. At this point, corrosion occurs, which is likely caused by corrosive gases. The heat treatment department will emit corrosive smoke, and one of the most corrosive components is SO2 smoke, which can quickly corrode the surface of the workpiece; Building dust is also a factor that causes rapid corrosion of metal surfaces. Ventilation in metal processing areas is the only way to solve corrosive gases. Removing corrosive smoke can solve the corrosion problem caused by it. Bimetallic corrosion Bimetal corrosion refers to the corrosion caused by Electron transfer between two different metals due to the presence of metal working fluid (conductive medium) in the process of processing. A metal working fluid with better inhibition of bimetallic corrosion can be used. Generally, metal working fluids with higher mineral oil content can avoid such corrosion, such as enhanced emulsions. Using waterproof grease or plastic film can also block surface contact between different metals. Demulsification When demulsification occurs, the instability of the emulsifier leads to the failure of the anti rust additive, which is prone to corrosion. When the emulsion appears water like or layered, the emulsifier may have failed or been destroyed. Under single machine conditions, the simplest solution is to pour old metal working fluid, thoroughly clean with efficient machine tool cleaning agent, rinse with fresh water, and finally add fresh metal working fluid according to the recommended concentration. The instability or demulsification of emulsions is often caused by high bacterial content. It is very important to confirm the cause of demulsification before taking measures. Additives may be used when processing the central tank system, such as using emulsifiers to re emulsify metal working fluids. If time permits, please take samples and send them to the metal working fluid supplier for testing; If time does not allow, testing can be conducted in containers (such as beakers) to determine which additives to use and the proportion of use.
