Preventing Rust & Corrosion with Coolant

Corrosion is a common problem machinists face. If you use a water-based cutting fluid, then your machine and workpiece will be exposed to water and likely develop (at least a small amount of) rust. It can damage expensive equipment, degrade your tools, and compromise your workpiece.

But there's good news! You don't need to complicate preventing corrosion. There a few things to keep in mind, and a few simple steps you can take to ensure it's never a problem.

Let's break it down!

Keep an Eye on Coolant Concentration

At Excision, we shout this from the rooftops! Regular testing of your coolant concentration is crucial. Coolant contains special corrosion inhibitors designed to prevent rust, but if your concentration is too low, these inhibitors become ineffective - there's simply not enough of them to do the job. However keep in mind that cutting fluid is not a rust protector - it is for cooling and lubrication.

A concentration below 5% often leads to rust. Go for a higher concentration, somewhere around 5-12% (your manufacturer will recommend a target concentration).

Use a refractometer to check your coolant regularly, and adjust the concentration accordingly. Don't let it get below 5%, because you're exposing yourself to the risk of corosion and bacteria!

Nobody wants a foul rotten-egg smell in the workshop, and a pallet-load of rusty workpieces.

Learn more about the importance of concentration testing (and how to do it) here.

Maintain the Right pH Levels

pH control plays a very important role in rust prevention. A pH between 8.8 and 9.2 will help deter rust and inhibit bacterial growth. If the pH falls below this range, bacteria will thrive which in turn produces acids that accelerate corrosion.

You can regularly check the pH of your coolant with test strips or a meter, and adjust as needed. If your pH is too low, it might be time to clean your sump out and recharge it with fresh coolant.

Good fun, we know, but sometimes you've got to do what you've got to do. Here's a handy guide on restarting your coolant tank.

Use Good Quality Water

Since water makes up a significant part of your coolant, it follows that the quality of your water matters greatly. Hard water, or water with high levels of chloride, sulfate, or nitrate, is aggressive. In turn, this results in corrosion.

Aim for water hardness below 250 ppm calcium carbonate. If your local water isn't ideal, consider using a blend of deionised or distilled water to improve coolant performance.

The truth is that many workshops in Australia do have reasonably soft water - except if you live in Adelaide (and maybe Brisbane). Learn more about the importance of water on your metal working fluids, including how it impacts foaming.

Manage Bacterial Growth

Unchecked bacterial growth can lead to rust by lowering the pH of the coolant. We'd recommend to a) keep your concentration in check and b) use biocides to keep bacteria under control and maintain coolant stability. Regular circulation and filtration will also reduce bacterial buildup (keeping it well aerated and removing any tramp oils).

If your coolant smells foul or shows signs of bacterial growth (like visible clumps or layers), thoroughly clean your sump and put in a fresh batch of coolant.

You can't ignore foul coolant (literally) and no amount of additives will fix a tank of fully-infested. If your sump’s gone rank, drain the tank. If you can smell it, it's too late to fix it.

We detail that more in this blog about bacteria in coolants - it has everything you need to know.

Choose the Right Coolant

Not all coolants offer the same level of rust protection. Mineral oils naturally provide excellent rust prevention, whereas synthetic coolants are less effective by nature (they have zero mineral oil, only synthetic polymers).

Newer neo-synthetic coolants like Excision's XDP3800 do have better rust prevention than synthetic fluids of days gone by. And they're great for a variety of reasons - clean, high lubricity, totally impervious to bacteria. However! Great corrosion prevention is not one of them. If the water is too hard, synthetic cutting fluids are not your friend - the water would need to be 0ppm for no rust (and that doesn't exist).

Natural mineral oil's composition is simply superior to any man-made substance at preventing rust. So if you need better corrosion protection, you should be using a water-soluble mineral cutting fluid.

Some Other Things to Note

Seasonal Considerations

Corrosion risks increase during hot, humid conditions. Australia's sweaty summer months (December-February) typically pose the highest risk. During these times, you could increase your coolant concentration slightly to compensate for increased humidity. We have a full break-down on managing your sump over the summer months.

Process and Material Awareness

Certain materials and operations are more prone to rust. Cast iron, for example, can rust quickly when stacked without spacers, creating galvanic cells that accelerate corrosion. Corrosion on yellow metals is also common (E.g. Brass, Copper). This is often due to your coolant being incompatible and reacting with the metal. Check with your manufacturer if you are unsure if your cutting fluid is compatible the materials you're machining.

Also, if you are storing a piece of steel after machining it, we always recommend that you coat it with a rust protector.

Regular Maintenance

Finally, a little routine maintenance can go a long way. Skim tramp oils regularly (either with a manual or continuous skimmer) as tramp oils encourage bacterial growth and rust. Also, recirculate your coolant daily to prevent stagnation, especially if machines sit idle. This can be a killer over the summer break.

Corrosion Doesn't Have to Be Hard

Corrosion is manageable if you're proactive. Monitor your coolant regularly, maintain the correct concentration and pH, and choose a coolant suited to your workshop's conditions. Good maintenance practices mean fewer headaches and a longer lifespan for your equipment.

And if you need help choosing the right coolant or maintaining your system? Our experts here at Excision are ready to help.