Degreasing metal parts — four methods, and when to pick which

Degreasing is removing oil and grease from a metal surface with chemistry and physics together: an agent that dissolves or lifts the grease, and motion, heat or pressure that carries it away. It is not the same as making the part "clean enough to look at". The goal is a surface the next step can bond to.

That next step is almost always something that depends on contact with bare metal: paint and powder coating, pickling and passivation of stainless, phosphating, bonding, or simply assembling parts that are meant to stay put. All of these fail silently on a greasy surface. The coating looks fine out of the oven, and lets go six months later. The passivation goes patchy where the oil was. The adhesive holds in the lab and not on the part.

That is why degreasing comes first in any pretreatment — and why it is so often the step that gets the blame when something later goes wrong, without anyone checking whether it was done.

1. The metal. Aluminium, zinc and brass do not tolerate strong alkali — the surface etches and turns dull or dark. They need a mildly alkaline or neutral agent. Steel and stainless tolerate stronger alkaline solutions, and often need them to shift dried-on oil. One and the same bath therefore does not suit everything that comes off the machines.

2. The oil. Thin coolant emulsion rinses off easily. Neat cutting oil is viscous and needs heat. Grease, wax and preservatives are worst, and need either time in a hot bath or mechanical help. If you know what was used in the machine, you know a lot about what it will take.

3. How dirty the part is. A light oil film is one thing. Oil with chips in it, soot from heat treatment or dried-on residue is another — then the degreasing also has to move solid particles, and the choice falls on a method with motion: spraying under pressure or a bath with circulation.

Spraying. Detergent is sprayed onto the part under pressure, from fixed nozzles in a machine or from a hand-held gun in a booth. The motion does the work: the jet lifts the grease and carries off chips and particles in the same pass. With hot water this is the most versatile method for the workshop, and the only one of the four that also removes solid residue. The drawback: the jet only reaches where it points. Parts with deep cavities have to be positioned right or turned.

Dipping. The part is lowered into a heated bath and left. Simple, and the bath reaches everywhere — into galleries and threads. The hotter the bath, the shorter the time needed. The drawback is that a still bath does not move particles; oil with chips in it settles on the bottom and the part can come up with residue on it. Circulation or ultrasound makes up for that.

Steam. Hot steam condenses on the part and runs off with light soiling. Gentle, little chemistry, and good for complex shapes. But only for light contamination — thick oil and grease defeat it.

Vibration. The part is set oscillating beyond the adhesion of the grease, so it shakes loose — with no chemicals. Interesting where emissions and waste are the problem. But the method removes what sits loose, not what has dried on, and it is not common in an ordinary workshop.

The detergent is not free, and neither is the spent detergent. A bath or tank becomes, over time, a mix of agent, oil, chips and dirt that has to be handled as waste — with packaging, transport and treatment. The cost depends on what chemistry it is, and how much of it.

Two things keep it down. Water-based agents instead of solvents: less to declare, less to ventilate, and the bath can be used longer. And separation along the way: an oil skimmer that takes the oil off the surface, and a strainer that catches the chips, let the same tank wash far more parts before it has to be emptied. That is the hidden economy of a closed washing machine — not the machine itself, but how rarely it has to be emptied.

Be honest about the limit at the same time: a water-based bath has a lifetime, and a tank that "still looks fine" can be finished. Set intervals for replacement, and keep to them.

The automatic parts washers we carry are hot-water spray machines: the parts sit in a rotating basket, heated water-based detergent is sprayed from fixed nozzles, and everything drains back to a tank with a strainer and provision for an oil skimmer. That is method one — spraying — packaged so it runs without an operator and gives the same result every time. Sizes run from bench model to machines that take several hundred kilos per basket.

For drums and barrels there are dedicated machines that wash the inside of 30–250-litre drums, one or four at a time. And for those who need solvent instead of water — for parts that must not get wet, say — there is one air-driven model with a service drum.

What we do not carry today is steam plants, vibration degreasing and manual high-pressure booths. If your production needs those, we will say so.

Wrong agent on the wrong metal. The most common mistake. Strong alkali on aluminium gives a dull grey surface that does not come back. Read the agent's data sheet — it says which metals it is made for.

Rinsing. Detergent residue on the part is as bad as oil residue for what comes next. Rinse with clean water where the process requires it, and dry.

Flash rust. A degreased steel part is bare and wet. It can start rusting within hours. Dry it, and apply protection if it is not going straight on.

Cold bath. Almost every water-based agent is made to work hot. A bath below working temperature means longer time and a poorer result, and the temptation is to raise the dose instead of the temperature. Do not — follow the data sheet.

Parts shielding each other. In a basket or a bath: if the parts lie tight, the faces between them are never hit. Load in one layer, and turn parts with cavities so they drain.