Understanding the cause and finding a solution
The Problem with Corrosion
In the metalworking industry, corrosion presents a serious problem and is a costly burden on manufacturers. Corrosion can destroy the dimensional integrity of a finished part and as well as parts of the machine itself.Additives are built into metalworking fluid formulations to help protect the metal of both the machine and parts from corroding.
The following provides an overview of the types of corrosion that may occur in a shop, reasons why this corrosion may happen and what you can do to address the problem.
Types of Corrosion
The definition of corrosion is the chemical reaction of a metal surface with its environment.
The most common type of corrosion is rust. Rust happens when the iron in ferrous metal oxidizes. The words corrosion and rust are often used interchangeably, however, they are not the same. Corrosion includes any metal surface, while rust refers specifically to the red rust most of us are familiar with.
In metalworking, another common type of corrosion is staining, typically seen on aluminum and copper. Additives used to help buffer and keep the pH up can and will attack aluminum causing a black stain.
Sulfur additives used in straight oils to protect the metal during heavy duty operations can and will react with yellow metals containing copper causing the surface to blacken similar to aluminum.
Causes of Corrosion
Controlling your coolant’s concentration is the most important thing you can do to avoid almost all problems in your machine, including corrosion. High quality rust preventatives are formulated with corrosion inhibitors that will protect metal surfaces from rusting and staining.
When concentration gets too low, there is not enough additive in the coolant to protect the surfaces of machined parts and the machine tool.
When the concentration is too high the amount of buffering additives might be too high as well and can attack and stain the finished parts and machine. This is where a refractometer is essential to maintaining your concentration.
pH Control of Fluid
Your coolants are designed to operate within a very specific pH range to provide the best protection on a wide variety of metals. If pH drops below 8.0, iron alloys will become susceptible to rusting and could be a sign of bacterial contamination. Water-based products are susceptible to bacteria, mold, yeast, and fungi through the water that is used and the “food” that is present in the form of additives.
Tramp oils, lubricants used for lubricating your machine and not directly in the cutting process, also provide food to bacteria. Tramp oils are straight oils and include slideway lubricants and spindle oils that are used to lubricant ball screws, machine ways, and spindle bearings in milling and grinding machines, lathes, and other cutting machines. As these oils build up in the coolant, the potential for bacteria also increases. As bacteria consumes the oils they let off acid causing the pH to drop.
Additional reasons for pH drop are poor concentration control, chip buildup in the sump, and contamination from outside sources.
Chlorides play a major part in the corrosion of ferrous metals. Iron alloys are particularly vulnerable to chloride ions which impact rust layers by contributing to the consequences of corrosion. The smaller amounts that are typically found in tap water will not cause a problem, however, some waters are naturally high in chlorides which can build-up in the sump over time due to evaporation.
Chips should be removed from the workspace and sump continuously. The metal-metal contact of the chips to the machine in the presence of water can cause corrosion.
Water on Surfaces
Compressed air is sometimes used to blow off surfaces of the machine. It is important to make sure compressed air is dry and that condensed water does not make its way into your machine. If you have water in the air line, it can wash away the protective coolant layer and cause corrosion.
Presence of Stray Current
Stray current corrosion occurs when current flows through paths other than what is intended. It is often referred to as stray current electrolysis and electrolytic corrosion. This happens when oxidation as an electro-chemical reaction, which involves the flow of electrons from one area (anode) to another (cathode) in the presence of oxygen and an electrolyte (the coolant). If the machine components aren’t properly grounded and stray current is present, anodic corrosion may occur on the surfaces of the machine. This type of corrosion is suspected when parts are not rusting but the machine is.
There are many other scenarios that can result in corrosion, which are dependent on the type of metal being affected. Contact the technical experts at KOOLRite for help in resolving corrosion issues in your shop.
Click the button below to download our Special Report on Corrosion Protection in Your Shop