Rust preventive oil is a liquid that leaves a film on a part right after machining, so rust does not start on the way to the next step, in storage or in transport. Which one to use depends on three things: how the part leaves the machine (wet, with oil or dry), where it goes and what comes after it, such as assembly or painting. In general, within the same type of film, a thicker one protects for longer and takes more work to remove before that last step.
Why does a machined part rust so fast?
A freshly machined part rusts within hours because it leaves the machine with clean, exposed metal, and water reaches it by several paths at once. Think of a glass of ice water that "sweats" in summer: the water running down the outside came from the air.
Rust needs three things: iron, water and oxygen from the air. The general name for this attack is corrosion: the metal reacts with its surroundings and slowly breaks down. In steel, the result is brown rust.
First, the metal. The tool strips away the oxide skin (metal already combined with oxygen), the dirt and the old oil from the surface. In air, plain steel soon grows a very thin oxide layer, but it has gaps and, with moisture, turns into porous rust that lets water through.
Stainless steel resists because it contains chromium, which forms a closed layer that rebuilds itself; even so, it stains if salt or plain steel dust stays on the surface.
Cast iron, common in blocks and housings, rusts even faster, because it has pores and graphite (the same carbon as in a pencil) that hold machining fluid and water.
This text covers steel and cast iron; aluminum, brass and copper follow other rules, and the conversation about them is with the Kelpen Oil Technical Team.
Next, the water. It reaches the part by four paths:
Fluid and wash water trapped on the part. The liquid that cools and lubricates the tool and the water from the washer stay trapped in threads, corners and blind holes (holes that do not go all the way through the part) after the rest has dried. This applies especially to water-based fluid, such as soluble oil (the white mix of oil and water).
Air humidity. Water vapor in the air deposits water on the metal, and salt and dirt on the part hold that water.
Condensation. In the early morning, or near an open door, the part cools below the dew point, the temperature at which water vapor in the air turns into drops. It is the glass that sweats.
Hands. Sweat leaves salt on the part. Salt holds moisture and makes the water more aggressive to steel; that is why rust shows up in the shape of a fingerprint.
While the part is in the machine, the cutting fluid protects it, as long as the water-based fluid is at the right concentration. When the part comes out, that protection ends quickly. Water-based fluid dries and leaves an uneven residue. Neat oil (cutting fluid made of pure oil, without water) leaves a film that holds off rust only for a short time.
Figure 1. Water reaches the machined part by four paths, and rust starts where it stays.
How this shows up in your plant: brown spots the next day, fingerprints marked in rust, a stain where the part rested on the pallet or on another part, and rust inside threads and blind holes.
For those who need the number: ISO 9223, which classifies how aggressive the air is to metals, estimates the "time of wetness" by the hours with relative humidity above 80% and temperature above 0 °C. It is a climate rule, for comparing places: salt and dirt on the part hold water even below that.
How does rust preventive oil hold off rust?
Rust preventive oil holds off rust like a raincoat: it makes it harder for water and oxygen from the air to reach the steel. It does this in two layers.
The first is the corrosion inhibitor, an additive (a chemical that reinforces what the oil does). It comes mixed into the preventive and attaches to the steel in the same application. Its molecules have one end that sticks to the metal and another that stays in the oil. They line up on the steel side by side, like the bristles of a brush, and get in the way of water and oxygen.
Some inhibitors also neutralize acids that reach the part, such as the vapor from acid baths in a plant (the section on the place explains).
The second is the film, the layer of oil or wax that stays on the part after application and plays the role of the coat.
The liquid that carries the film to the part changes from product to product. There are three types:
Solvent-based: the solvent is a thin liquid that carries the product into crevices and then evaporates, leaving the film. That is why some preventives need a wait before the parts are stacked.
Solvent-free: oil or wax with additives, with nothing that evaporates; what goes on the part stays as the film.
Water-based: the water evaporates and the film stays.
Rust preventive oil is also called a temporary preventive. The word refers to removal: the film comes off when you want, with the right product, before assembly or painting. How long it protects depends on the film and on where the part is kept.
The preventive stops new rust, but it does not undo rust that has already started. That is why the part must reach it clean.
Figure 2. The inhibitor sticks to the steel and the film covers everything, like a coat against water and oxygen.
For those who need the standard: ISO 6743-8 classifies temporary corrosion protection products (family R) by application and by type of film. It explains that "temporary" refers to the possibility of removing the product; how long it protects is another question. The same standard leaves out vapor-phase inhibitors (VCI), which protect through the vapor they release inside the packaging.
What types of film does rust preventive oil leave on the part?
Rust preventive oil leaves one of four types of film: oily, dry-film, waxy or waxy dry-film. Think of what is left on your hand after three household products. Cooking oil leaves a thin, wet layer. Petroleum jelly leaves a thick, soft layer that stays wet. Nail polish dries and stays thin.
A layer can be dry and thin, like nail polish, or wet and thick, like petroleum jelly: "dry" and "thick" are different things. That is why the film is described by two questions: is it wet or dry to the touch? Is it thin or thick? With those two questions, the four types look like this:
Oily: wet and thin. It is the easiest to remove, used between steps and in a covered warehouse.
Dry-film: dry to the touch after the solvent or the water evaporates, and thin. The part can be handled and stacked without leaving marks.
Waxy: contains wax, is thick and stays wet or soft to the touch. Dry to the touch, which makes handling and transport easier.
Waxy dry-film: thick and dry to the touch. Used on bars, profiles and finished parts that go into long storage or transport.
The rule that ties it all together holds in general: within each type, the thicker film protects for longer and takes more work to remove. The inhibitor also matters: two films that look the same in your hand can protect differently.
Figure 3. The four films in two questions: wet or dry to the touch, thin or thick.
Two more films are left aside in this text, because they get too thick or too hard to come off before assembly: the grease-like film, a thick paste like grease, and the protective varnish, made of hard resin that is difficult to remove.
When should rust preventive oil be applied after machining?
Apply rust preventive oil in the same shift in which the part leaves the machine, before condensation or handprints build up.
What changes the choice most is the water left on the part. Think of a bandage put on wet skin: it peels off and traps moisture underneath. Water trapped under an ordinary film does the same and keeps corroding, now hidden.
For a wet part there is the water-displacing preventive. It is a liquid that wets steel more easily than water does, gets under the drops, lifts the water off the surface and pushes it out, leaving the film in its place.
Figure 4. Under an ordinary film, water stays trapped and rusts; the water-displacing preventive gets under the drop and pushes it out.
With that, the part leaves the machine in one of three situations:
Wet part from water-based fluid or from the washer: use a water-displacing preventive, applied directly, without drying the part first.
Part with neat oil: it already has an oil film, which protects for a short time. For storage, ask the Kelpen Oil Technical Team whether the preventive can go on top or whether the part must be cleaned first.
Dry, clean part: a non-displacing preventive will do, as long as the part is free of chips (the metal shavings from machining), dust, rust or grease.
Water displacement is a property of the product, which can come with more than one type of film. The water the displacing product pushes out sinks to the bottom of the tank and has to be removed from there, as you will see in the application.
How this shows up in your plant: patches of rust under the film, days later, on a part that "was protected".
Which rust preventive oil should I use on each machined part?
The right rust preventive oil comes from crossing the three things from the start: how the part leaves the machine, where it goes and what comes after it.
| If the part… | Type of preventive | Why |
|---|---|---|
| Leaves wet and returns to another operation within a few days | Water-displacing, oily film | Removes the water and leaves a thin film that comes off easily |
| Leaves dry and waits in a covered warehouse | Oily film | Protects from air humidity and comes off easily |
| Leaves wet and goes to storage or transport with handling | Water-displacing, dry-film or waxy | Removes the water and makes handling easier |
| Leaves wet and goes to long storage | Water-displacing, waxy or dry-film | Removes the water and protects for longer; the Kelpen Oil Technical Team recommends the thickness for your storage time |
| Is already dry and goes to long storage (bars, profiles, finished parts) | Waxy dry-film | Thick and dry to the touch, protects for longer |
| Leaves dry and goes to transport with handling | Dry-film or waxy | Makes handling easier and does not require removing water |
| Leaves with neat oil | To be defined with the Kelpen Oil Technical Team | The neat-oil film holds little; the Technical Team says whether the preventive goes on top or the part is cleaned first |
What is missing is what comes after the part. If the next step is painting, welding or bonding, a thinner film usually costs less than removing wax from every part.
The place matters too. Electroplating deposits a layer of zinc, chromium or nickel on the part with electric current; acid pickling is the bath that removes the oxide skin from steel. Near those baths, acid vapor in the air crosses an ordinary film and reaches the steel faster. There, the film has to be designed for an acid environment.
Kelpen Oil formulates the Keen Prot line of temporary anticorrosive preventives, with oily, dry-film, waxy and waxy dry-film options, water-displacing options and options for acid environments, such as electroplating. The Kelpen Oil Technical Team recommends the product for each part and storage time.
How do I apply rust preventive oil to machined parts?
The most uniform way to apply rust preventive oil is immersion: the part, or a basket of parts, is dipped in the preventive tank and drains. Brush, roller and spraying work for large parts or small quantities. For immersion:
Remove the chips and the excess fluid. Trapped chips hold water and puncture the film.
Dip the whole part, turning it if there is a blind hole or thread, so the preventive gets in where the water got in.
Let it drain. With a solvent-based preventive, wait for the solvent to evaporate before stacking or packing, because the film only forms after that.
With a water-displacing preventive, use a tank with a drain (an outlet at the bottom) and drain the water pushed out of the parts regularly. It is heavier, sinks and, if it builds up, goes back onto the next part.
If the preventive turns cloudy, the soluble oil residue that the parts bring in is building up in the tank; talk to the Kelpen Oil Technical Team.
Handle with gloves after application. A bare hand removes the film and leaves sweat salt.
The solvent evaporates into the plant air, and each product has a flash point: the lowest temperature at which its vapor ignites for an instant when a flame passes nearby. That is why the tank stays in a ventilated place, away from flames and hot parts. The value and classification of each product are in its safety data sheet.
A part that has just left heat treatment or a heated washer waits to cool before receiving a solvent-based preventive, because solvent vapor over it ignites more easily. The wait is only the time to cool, never from one shift to the next, so the part does not rust in the meantime.
How long does rust preventive oil protect the part?
The protection time depends on the film and on where the part is kept, and no laboratory test converts that into exact days. It is like the paint on a gate: the same paint lasts years indoors and far less near the sea.
The same film lasts much longer in a dry, closed warehouse than near the sea, in an open warehouse or in a container that heats up by day and cools at night, because every cold night brings new condensation. The packaging also counts: damp cardboard or wood against the part brings the water back.
The test used to compare preventives is the salt spray test. A steel panel coated with the preventive sits in a closed chamber, in a mist of salt water, until rust appears, and the result comes out in hours. Those hours are good for:
comparing one preventive with another;
checking whether a batch came out the same as the approved one.
They are not good for turning into a storage time. The test standard itself warns that the result rarely matches what happens in a real environment.
Figure 5. The hours in the salt spray chamber compare preventives; the storage time depends on the place.
In practice, the real storage time is set in three steps:
Choose the film by where the part is going, as in the selection table.
Test a pilot batch in your own storage and your own packaging.
Inspect the parts on fixed dates and note what you see.
For those who need the number: the salt spray test follows ASTM B117, with a 5% sodium chloride solution and a chamber at 35 °C. The significance section of the standard says that the result alone rarely correlates with performance in a natural environment.
How do I remove rust preventive oil before assembly or painting?
Rust preventive oil comes off with a degreaser or a solvent, and the choice depends on the film. The reason to remove it: residue of oil or wax keeps paint and glue from sticking and contaminates the weld. A part going to painting, welding, bonding or another surface treatment must be completely clean.
Think of a greasy pan: water alone does not clean it, and water with detergent does. In the plant, the detergent's role goes to the alkaline aqueous degreaser, a water-based cleaner that acts like soap. Soap is alkaline, the opposite of lemon, which is acidic. The degreaser sits on the soap side.
Each film calls for its own route:
Oily: alkaline aqueous degreaser or solvent.
Dry-film: alkaline degreaser or solvent, depending on the product.
Waxy and waxy dry-film: they usually ask for more, with solvent or a heated alkaline degreaser (heat softens the wax) and more contact time.
That is why choosing the film already looks ahead to the last step of the line. For this cleaning, Kelpen Oil also has a line of industrial degreasers.
Frequently asked questions
Doesn't cutting fluid already protect the part? It protects while the part is inside the machine, with the fluid at the right concentration. With water-based fluid, after the water evaporates a thin, uneven residue is left; with neat oil, the film protects for a short time. For storage and transport, the preventive is necessary.
Is rust preventive oil flammable? It depends on the liquid it carries: a solvent-based one usually has a lower flash point. NR 20 is the Brazilian regulation, from the Ministry of Labor and Employment, for flammables and combustibles. It calls a liquid with a flash point up to 60 °C flammable and one above 60 °C and up to 93 °C combustible. The classification of each product is in the SDS, the safety data sheet under NBR 14725:2023 (the Brazilian standard for chemical product safety information, formerly FISPQ), which comes with the product. Check the sheet before deciding where to store and apply it.
Do I need to remove rust preventive oil before painting? Yes, in general. Residue of oil or wax keeps paint from sticking, and the painting system defines the cleanliness level. The preventive comes off with a degreaser or a solvent, depending on the film.
Glossary
Corrosion inhibitor: additive whose molecules stick to the metal and get in the way of water and oxygen.
Films: oily is wet and thin; dry-film is thin and dry to the touch after the solvent or water evaporates; waxy contains wax, is thick and stays soft; waxy dry-film is thick and dry to the touch.
Water-displacing: a preventive that wets steel more easily than water and pushes it off the surface.
Salt spray test: a test that compares preventives in a chamber with a salt-water mist; the result comes out in hours.
Flash point: lowest temperature at which the product's vapor ignites for an instant when a flame passes nearby.
Talk to the Technical Team
The Kelpen Oil Technical Team evaluates how the part leaves the machine, the metal, the cutting fluid in use, the storage time and place and the next step, and recommends the right preventive. If the part keeps rusting with the preventive or the cutting fluid already in use, tell the Technical Team what shows up on the parts and in the tank. It says whether the case calls for an analysis in the laboratory in Arujá (SP), which is a paid service.
Kelpen Oil checks its preventives in its own laboratory in Arujá (SP), which performs 25 analyses; no batch ships without the laboratory's approval. For preventives, the laboratory looks at these points:
| What we measure | What it reveals |
|---|---|
| Salt spray resistance (ASTM B117), when the product calls for it | How well the film resists accelerated corrosion, to compare preventives |
| Density | The mass of a volume of the product, which changes if the formulation changes: it shows whether the batch is the same as the approved one |
The tests, standards and instruments are in the text about the Kelpen Oil lubricant laboratory.
Before switching preventives, note three things: how the part leaves the machine (wet, with oil or dry), how many days it waits and where, and what comes after it. Those are the three answers that decide the film. Save this article or send it to whoever handles machining and shipping at your company.
