Compounds for Aluminium: What the Chemistry Has to Do That Media Cannot

The media cuts aluminium (aluminum in US spelling) — and when a buyer searches compounds for aluminum, that is the same product. The compound decides whether the cut leaves a surface you can sell.

That split is the whole subject of this page, because aluminium is the metal where the chemical half of the process is most easily got wrong — and where the failure looks like a media problem, so the buyer changes the charge and the result does not improve.

Three things about aluminium set the compound requirement apart from steel, and each one rules out a class of product.

Why a steel compound fails on aluminium

A compound formulated for steel is built around a hard, work-hardening surface that tolerates an alkaline bath and a strong cutting chemistry. Aluminium is none of those things.

1. It is amphoteric — it reacts with both acids and alkalis. A strongly alkaline compound attacks the aluminium itself, not just the oxide on it. The visible result is a darkened or blotchy surface that no amount of polishing lifts, because the metal has been chemically etched rather than mechanically finished. This is the single most common cause of a "the aluminium went black" complaint, and it is a compound problem that gets blamed on the media.

2. It oxidises within minutes. Freshly cut aluminium forms an oxide layer almost immediately on exposure to air and water. That is not a defect, but it does mean the compound has to keep the surface in a consistent state between the cutting stage and the brightening stage — on a wet process, that means the bath has to be managed rather than left.

3. It smears rather than chips. Where steel cuts and throws a chip, aluminium deforms and smears. The smeared layer traps compound and swarf, and a compound that does not keep the swarf in suspension lets that mixture re-polish the surface back dull. This is the aluminium version of the grey haze that plagues stainless.

What follows: the compound for aluminium is chosen for what it does not do — does not etch, does not leave a residue that oxidises, does not hold swarf against the surface — before it is chosen for how fast it cuts.

The four requirements, in the order they matter

The third one is the one that costs most and gets checked least. Aluminium holds water in exactly the places a compound residue is hardest to rinse — blind holes, the gap under a fastener, rolled edges. A part that looks perfect at goods-in and shows spotting a week later failed at the rinse, not at the polish.

The time budget the compound is working in. A cutting cycle on aluminium runs 10 to 20 minutes for a light deburr and longer where stock has to come off; a brightening cycle runs 2 to 8 hours. The compound has to hold the surface in a consistent state across that whole span — on the long brightening cycle in particular, a bath that is drifting shows up as a colour change rather than as a roughness change, and colour is what the buyer sees.

Two aluminium-specific traps

Trap 1: the compound is fine, the water is not. In a hard-water area, the calcium in the water reacts with the compound and leaves a film that reads as a compound fault. Before changing the compound, check the water — it is a cheaper test than a new drum of product.

Trap 2: a "universal" compound run on both steel and aluminium. A shared charge and a shared compound is the arrangement that produces the most confusing results, because steel swarf in an aluminium bath is contamination and aluminium swarf in a steel bath is harmless. If both metals run on one line, the compound and the charge should be changed between materials — and the change should be written down, because it is the step that gets skipped on a busy shift.

Where the compound is the wrong lever

Being straight about this saves a lot of product being bought and returned:

• The part is black because it was over-cut. A compound cannot restore metal that has been removed. If the surface is etched or the edges are rounded past the drawing, the answer is upstream, not a different drum.
• The part is dull because the media is spent. Media life runs 20 to 200 operating hours, and the replacement signals are a finish that stops improving, a matte grey haze, and a cycle about 20% longer. A new compound over a spent charge will not recover the cut.
• The part has an internal burr. No compound reaches a cross-hole or a thread root; that is a process question. The same physical rule that governs media — media should never be larger than about one third of the smallest feature, so a 3 mm slot needs media below 3 mm — governs what the compound can reach, because the chemistry only acts where the medium takes it.
• A thin wall. Below roughly 0.5 mm the risk on aluminium is distortion from heat and handling rather than chemistry, and the guide is a process family change rather than a different compound; the threshold is a starting point, so confirm it against your section on a trial rather than treating it as a rule.
• The requirement is isotropic. A loose charge produces a non-directional finish from the media, not from the chemistry.

What we actually stock for aluminium

We manufacture the media and compounds that run in our machines, and the aluminium-specific part of the range is small and specific — which is the honest way to describe it:

• A green liquid polishing compound / grinding oil for aluminium and metal parts, used as the cutting-stage chemistry.
• An aluminium grinding compound in green liquid form, for the same stage where a heavier cut is specified.

Both are supplied in 20 kg drums, which is the working unit for a shop running a machine rather than a lab running a trial. Alongside them the general range covers metal, stainless and acetate fluids, an abrasive solution and an abrasive brightener, a degreasing and anti-rust powder, and an electrolytic agent — all at 20 kg barrel or 50 kg drum scale.

What we will not do is call one of them universal. A compound that runs well on steel and aluminium at the same concentration is not a product we would claim, because the pH band that suits steel attacks aluminium.

What to send with an aluminium compound enquiry

1. The alloy, not just "aluminium" — 6061, 7075 and a casting alloy behave differently in the same bath, and the compound's pH band has to suit the one you run.
2. The stage the compound is for: cutting, brightening, cleaning, or corrosion protection between stages.
3. The process: wet or dry, and which machine family.
4. What the surface has to be after — a roughness figure and a colour requirement if the part is plated or anodised later.
5. The water, if it is hard or from a borehole; it changes the rinse behaviour as much as the compound does.
6. The order scale. Compounds ship at 20 kg barrel or 50 kg drum depending on the product, and a standard machine ships in 25 to 35 days from the deposit with the media and compound able to travel in the same case — so the first trial can run on arrival rather than waiting for a separate consumable order.
7. The existing problem, if there is one: what the part looks like now, and at which stage it changed.

FAQ

Can I use a stainless polishing compound on aluminium? Not safely, and it is the most common cause of a disappointing aluminium trial. A compound formulated for a hard, work-hardening surface is typically more aggressive and more alkaline than aluminium tolerates. The result is etching or smearing, and adding cycle time makes both worse rather than better.

Why do aluminium parts go black during polishing? Almost always a chemical reaction rather than a mechanical one — a bath outside the aluminium-safe pH band, or a compound that attacks the metal instead of the oxide. The test is to run a scrap part of the same alloy in the suspect bath and watch it for 15 minutes; a colour shift identifies the chemistry rather than the media.

How do I stop water spots on aluminium? Dry the part in a centrifugal dryer rather than by air, rinse before the water can carry residue into blind features, and check whether the water itself is hard. A compound that rinses clean in soft water can leave a film in hard water, and that film is what oxidises.

Does compound choice change the achievable brightness? It changes whether you reach the brightness, more than how bright it can be. The media sets the surface profile; the compound decides whether the swarf is carried away so that the profile is what you see, rather than a smeared version of it.

Are compounds for aluminium different between alloys? The pH band is the same class of constraint, but a casting alloy with porosity and a wrought 6061 part behave differently in the same bath — the casting holds bath in the pores and needs a different rinse. Give the alloy in the enquiry, because it changes the rinse and the drying as much as the compound.

Max Xu, Sales — Xiamen Jin Tai Jin Grinding Technology Co.,Ltd.

No. 31, Xinchang Road, Xinyang Industrial Zone, Haicang District, Xiamen

info@surface-polish.com

This article describes how an aluminium compound is specified; it is not a claim about any specific alloy's achievable result. A finishing process is set by the material, the stock to be removed and the finish required, so the compound and concentration for a specific part are confirmed against a trial on that part. We manufacture finishing equipment and the media and compounds that run in it; we do not offer a job-shop finishing service.