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l Rust: Most Frequently Asked Questions -

I have rust occurring just minutes after I’ve machined a part. How can I prevent it?

If you check, you may find that your coolant concentration isn’t strong enough. There is enough rust preventive in fresh coolant to provide “in-process” rust protection. Of course, this doesn’t mean you can half-machine a part, set it on the shelf, and expect it to be rust-free two months later. However, parts going from one operation to the next should be able to set for a reasonable length of time without rusting. The fact is, when your coolant gets old, the rust preventive wears out and a stronger coolant mix may be necessary to provide the same rust protection you experienced when the coolant was fresh. So, if you are getting rust, first check the coolant concentration; then, examine how dirty your coolant has become over the months it has been in the machine. Dirty coolant can contain metal fines that interfere with good rust protection. Also, mineral build-up from your water supply can contribute to rusty parts.

If I coat my finished parts with a rust preventive, how long will it protect against rust?

Rust preventives go hand-in-hand with most coolant manufacturers’ product offerings. The type of rust preventive you use is dependent on the amount of film you want on the part and the length of protection needed.

Water-based rust preventives will usually leave behind a very thin and imperceptible film. This type of rust preventive is good for short-term or in-process rust protection. The biggest limitation here is that water-based rust preventives are not the best choice where humid conditions might occur. You can expect 30 to 60 days of rust protection from water-based rust preventives.

Solvent-based rust preventives offer humidity protection not found in most water-based products. Depending on the amount of solids vs. solvent, your film thickness will vary from imperceptible to slightly noticeable. Solvent-based rust preventives function much like paints. The solvent evaporates leaving behind a rust-protective coating. Three-to-six-months protection is typical for solvent-based rust preventives.

Oil-based rust preventives will usually leave a slight, but noticeable film on your parts. The oil acts as a humidity barrier and the film will often contain several types of antioxidants and fingerprint suppressants along with other ingredients. Oil-based rust preventives provide exceptional protection lasting a year or more.

Now, here’s the disclaimer: Each of the rust preventives mentioned have limitations based on how the film is applied, cleanliness of the parts being coated, and the environment in which the parts are stored. Just as you wouldn’t store parts coated with water-based rust preventive outside, you can’t expect freshly-machined parts coated with quality rust preventive to provide a year’s protection if they’re stored next to an acid bath in a plating shop!

What precautions can I take to prevent parts from rusting either just after machining or after even a month or more?

Sometimes customers dip parts immediately after machining into a stronger-than-usual coolant mix. Then, when that “dipping coolant” starts to get dirty or it’s the end of the shift, often the coolant is simply dumped into the sump. This might sound like a good idea, and it does work – but – let me explain why this isn’t such a good practice. Let’s say the coolant is designed to run in your sump at 5 percent to 10 percent concentration. That means that all the ingredients are measured out by the manufacturer to be safe for use at these concentrations. Now, if you double that concentration to use the coolant as a rust preventive, you are forcing your operators to stick their hands into a potentially harmful chemical mixture.

In addition, when misused this way, the coolant is NOT cost effective. I have to ask, why use a coolant that contains lubricants, emulsifiers, surfactants, and rust preventives to do a job that a rust preventive alone will do better? You would be money ahead dipping these parts in a water-soluble rust preventive designed for just such an application.

What is the best type of rust preventive for my application?

The answer to this question comes with another question: What is going to happen with the parts after they are coated with rust preventive? Are the parts going into long-term storage, or will the parts require long distance shipping? If yes, then an oil-based rust preventive is the best choice. Oil-based rust preventives do not always have heavy oil dripping characteristics. These products can be relatively dry if the coating is applied properly. There are also heavy wax-type products available for this type of application.

If your customer demands parts that can go directly into an assembly operation, then you need a water-based rust preventive that leaves an imperceptible film and usually does not require cleaning off before assembly.

When a thin, dry film is required in a more humid environment, you would be wise to choose a solvent-based rust preventive.

How should I apply the rust preventive?

The answer to this is somewhat dependent on the size of the parts. If possible, parts should be dipped as this ensures the entire part is coated. After dipping and draining, only a thin film should remain on the part and the balance of the rust preventive will flow back into the reservoir. Most rust preventives can also be sprayed or brushed on metal surfaces.

How can I remove my rust preventive when I’m ready for parts to go into assembly?

Usually, rust preventive suppliers will have a cleaner available to remove the rust preventive when needed. Generally, a mild alkaline cleaner (Industrial type “Top Job” or “Mr. Clean”), at 1.0 percent to 2.0 percent, will remove an oil-based or solvent-based rust preventive. I always suggest a low concentration “water-based” rust preventive in the rinse stage to prevent flash rusting. Water-based rust preventive can be easily removed with a weak solution of water-based rust preventive in hot water. If these parts are dried quickly, flash rusting should not occur.

As I mentioned, each approach to rust prevention has its limitations. I recall a customer once asked me, “Is there a rust preventive that doesn’t leave a film, that will protect my parts for a year, and not be affected by humidity or salt during long ocean voyages?” “Yes,” I said, “it’s called paint!”

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