Every coolant on the market today has built into it a certain level of rust protection. Some coolants have better rust protection than others. A quality coolant from a reputable manufacturer will provide adequate “in-process” rust protection. In the beginning we start with fresh coolant in a clean machine. This means that when the coolant is mixed at the recommended concentration, usually 5%-7%, your freshly machined parts should not rust between operations. Here are some questions to answer:
Have you checked the coolant concentration today?
This question is often the easiest to overlook. What is the coolant concentration? If you don’t check this first, it’s like calling the TV repairman when the television doesn’t work only to find that your cat pulled out the plug. Most coolants perform well between 5.0% and 7.0% depending on the coolant and the demands of the operation. When the concentration dips to 3.0% it makes sense that your rust protection won’t be as good. So before you do anything else check the coolant concentration.
Is there a preventive maintenance (PM) program for coolant?
When was the last time your coolant was changed? Removing tramp oil and metal fines will make your coolant last longer. Note however, that it will not make the coolant last forever. Your coolant might look great; but you can’t see everything that’s going on in the coolant. You can have bacteria growing but not have a strong odor. There may be metal fines too small for your filter to remove. Perhaps you have other contaminates such as dirt, water impurities or coatings that were on the castings before you started to machine them. As your coolant ages, the contamination levels increase while some of the coolant’s ingredients wear out; therefore, your coolant concentration may check out okay, but worn out rust preventive and increased contaminate levels will still cause rust on your parts.
The solution here is to have a regular PM on all the machine sumps. Depending on the coolant type, and your ability to remove contaminates, this coolant sump PM could take place every three to six months or every year or two. Fresh coolant at the proper concentrations will not cause rust problems, unless you have a water problem.
What is the condition of your water? Has it changed recently?
Hard water is a concern because the minerals will interfere with the coolant as well as cause machine surfaces to wear out prematurely. You might think, “Our water is fine, it’s city water and I know it’s not hard.” This may be true. The city provides water that is 7 grain hardness. Usually, problems with hard water don’t occur until the water is 25-30 grain hardness and above.
Let’s say you charge a new machine with fresh coolant and the water is at 7 grain hardness. Within four weeks you have added an equal portion of make-up coolant to that of the original charge. Since the water evaporates and the minerals don’t, after just four weeks you now have 14 grain water in your sump. In a very short period of time you are working with the same hard water problems as the fellow who uses well water. Coolant contaminated with very hard water will cause rust on your freshly machined parts. It’s like taking a handful of very fine sand and throwing it on the machined parts; wherever the sand lands it acts as a seed for rust to start.
The solution to this problem is to improve the quality of your water. Reverse osmosis (RO) or de-ionized (DI) water systems are well worth the investment. Not only will you reduce the problems caused by hard water but you’ll also see a reduction in your coolant usage.
Is there rust on the castings from the foundry?
Occasionally, parts are received from the foundry with rust on them. If these parts are machined without being cleaned first, those rust particles will end up in the coolant. The filtration system may remove most of this iron oxide (rust) but very fine particles will stay suspended in the coolant; especially, if the rust particles get oil attached to them. The oil wants to float and the metal fines want to settle. The compromise is they stay suspended. Over time the accumulation of suspended rust particles actually causes the particles to “plate out” on machine surfaces and machined parts. It’s not that you can’s machine parts with rust on them, you certainly can. Just be aware of what may happen over time and be prepared to change your coolant more frequently.
Have you added any “special” additives to your coolant lately?
This could add to your rust problem. As coolant ages bacteria starts to grow in it. Your coolant undoubtedly contains biocides to retard bacteria growth. Biocides do wear out in time and occasionally the addition of a small amount of biocide “tank-side” will eliminate odors and rejuvenate your coolant and this is good. However, some people mistakenly think that biocide for metalworking applications is nothing more than “bleach.” While bleach kills germs and bacteria in your weekly laundry, bleach has no place in a machine sump. Even at low doses bleach will cause rust on machine surfaces as well as on freshly machined parts. To see this for yourself, experiment by putting a handful of cast iron chips in a cup and add bleach. Within 15-20 minutes, you will have a mass of rust.
Finally, how’s your weather?
Sometimes its just “that time of year” when humidity is high and the rust preventive in your coolant isn’t enough to provide the “in-process” protection you require. The solution is to add a little extra coolant to the sump or perhaps dip the parts as they come off the machine. While this means a little extra work, it’s much easier than trying to remove rust from the inside of a hydraulic valve. Adding about 1% of a water-based rust preventive will also provide extra rust protection between operations. With this suggestion, please be sure the coolant and the rust preventive are compatible before mixing a whole sump.
