Rusty brake discs: when is it normal and when is it not?

Brake discs rust quickly because they are made of unprotected cast iron. A car body is painted and galvanised, a brake disc is not, because a coating on the braking surface would interfere with braking. As soon as the disc gets wet from rain, a car wash or damp air, a thin brown layer of rust soon forms. The ANWB, the Dutch motoring organisation, puts it briefly: brake discs can rust, and normally the brake pads wear that surface rust off again. That is why you see the rust mainly after a night outside, after a weekend without driving or after a drive in the rain, and why it has gone after the first few stops.

Look at the braking surface after you have braked firmly a few times. If it is smooth and metal-coloured again, it was surface rust. If dull patches, small pits or a rough edge remain, the disc is damaged. This is how we tell the forms apart in the workshop:

Workshop overview based on the explanations of the ANWB (brake discs), Ferodo (brake judder) and the RDW APK manual.

If the car has been standing for weeks, drive the first kilometres gently and brake firmly a few times in a safe place, so the pads scrub the rust layer off. The ANWB advises this if the squealing or scraping caused by rust does not go away by itself. If the noise persists, have the brakes cleaned or checked. On a car that has stood for months after a wet spell, the brake pads can rust onto the disc. You notice that as a loud bang when you drive off, and sometimes as a vibration when braking in the days afterwards, because the pad has left an imprint on the disc.

1. First drive after standing: drive off gently and listen. A light scraping is normal and disappears.
2. Brake firmly a few times: on a quiet road, with space behind you. This scrubs the surface rust off.
3. Then check the braking surface: through the spokes of the wheel you can see whether the disc is shiny and smooth again. A narrow lip is fine, pits are not.
4. If the car keeps shaking or squealing: have the brakes checked. We measure the thickness and the run-out of the disc and check whether the pads and the caliper still move freely.

If your car regularly stands for longer periods, preferably do not leave it with the handbrake on when it is wet, so the rear pads or brake shoes do not rust onto the disc or drum. Leave the car in gear or in P. You can read more about what to check after a long period standing in our article about a car that has been left standing.

An electric or hybrid car does much of its braking with the electric motor, so the mechanical brakes are used little and the rust is scrubbed off less often. The ANWB writes that the brakes of an electric car wear less quickly because you can brake on the electric motor, but that the mechanical brakes can be troubled by rust. The ADAC, the German motoring organisation, reports the same: because the mechanical brake comes into action less often, there is a risk of rust, and drivers do well to clear the brakes regularly by braking firmly. According to measurements by Green NCAP, published by the ADAC, electric cars recover up to 22 percent of the energy drawn from the battery through regenerative braking. For that share of the braking work, the brake discs have nothing to do.

Tesla gives the same advice in the Model 3 owner’s manual: press the brake pedal regularly so the mechanical brakes are used and the pads and discs dry out, to prevent rust and corrosion. Tesla also specifies that the brake calipers should be cleaned and lubricated every year or every 20,000 km where roads are salted in winter. On an electric car we therefore pay extra attention at every service to rust and to calipers and slide pins that are sticking. How that works and what you can do yourself, you can read in our article about brake maintenance on an electric car.

Squealing or light scraping the first time you brake after rain or standing is normal, and according to the ANWB it often disappears after a few firm stops. Shaking is not normal. If you feel a vibration in the steering wheel or the pedal when braking that does not go away after a few drives, that often comes from a thickness variation or run-out of the disc. Brake manufacturer Ferodo explains that the pads then lose and regain contact with every revolution. Pitting and the imprint of a pad that has rusted on are two of the causes. You can read more in our article about a car that shakes when braking. If a squeal persists that is not caused by rust, see the causes of squealing brakes.

Can you still drive? With surface rust and a light squeal: yes, drive gently and brake firmly a few times. With a scraping noise that persists, a car that pulls to one side when braking or a hot wheel after a short drive: no, have the car checked before you drive on. Not sure? Call us and we will schedule a short brake check. If nothing needs doing, we tell you that too.

At the APK, rust on a brake disc is only a fail when the smooth, uncorroded part of the braking surface per side, around the whole circumference, is less than 50 percent of the maximum width of the brake pad. That is how it is set out in the APK manual of the RDW, the Dutch vehicle authority, under article 5.2.31. A narrow rust lip is therefore no problem, a disc that has rusted away over more than half of its surface is. In addition, the inspector fails the car if the disc is so worn that breaking is unavoidable. Fine heat cracks in the braking surface are not a fail according to the manual.

Please note: the APK manual does not give a minimum thickness in millimetres. The inspector judges the braking surface and the risk of breaking by eye. The minimum thickness stamped on the disc (MIN TH) is a manufacturer’s requirement, not an APK requirement. That is why a disc can pass the APK and still be due for replacement. If you have received an advisory or a fail on the brakes, we repair it. If we repair the items that failed, we will also arrange the retest. We will tell you the cost of the repairs and the retest in advance.

We replace a brake disc when it is below the manufacturer’s minimum thickness, has too much run-out or has pitting that braking no longer removes. Rust on its own is therefore not a reason. This is how we assess it: we measure the thickness of the disc with a vernier caliper at several points around the disc, where the lowest value counts, and compare that with the minimum thickness stamped on the disc. Then we look at the run-out, at pitting and at the width of the rust lip. We also check whether the caliper and the slide pins move freely, because a sticking caliper is often the reason a disc rusts away on one side.

If the disc is still thick enough, flat and free of pits, it stays on. If it is worn or pitted, we replace the discs and the brake pads together per axle, left and right at the same time. The ANWB writes that on the same axle the left and right discs must be replaced at the same time, otherwise the car brakes unevenly, and that new discs go together with new brake pads. We do not skim brake discs. A disc that is too thin, warped or pitted, we replace. With modern discs that is usually better value than skimming, and you know for certain that the thickness and braking performance are right again. What you can expect from us and how we assess your brakes, you can read on our page about brake replacement in Rotterdam. We tell you one price in advance, based on your registration number, and you can see the old parts.

No, a thin layer of surface rust after rain or standing is not dangerous and wears off within the first few stops. It only becomes dangerous when the rust has damaged the braking surface, with pits or a wide rust lip, or when the disc brakes less as a result. Then the disc has to be measured and often replaced.

Brake firmly a few times on a quiet road. The brake pads scrub the surface rust off, which according to the ANWB is the normal way. Do not use rust remover, a wire brush or sandpaper on the braking surface and do not spray oil or lubricant on the disc. If the rust stays, it is too deep and the disc has to be measured.

No. Oil, grease or penetrating oil on the braking surface ruins the brake pads and makes the braking unpredictable. Only the parts that do not brake, such as the hub and the caliper slide pins, are treated by the garage with the correct product. Keep the braking surface clean and dry.

There is no fixed number of days. A weekend or a holiday of a few weeks is usually no problem for a car parked outside, the pads wear the rust off afterwards. After months of standing in a damp environment, the rust can eat deeper and the pads can rust onto the disc. In that case, do not leave the car on the handbrake and drive it a good distance now and then.

The front brakes do most of the work when braking and therefore keep scrubbing themselves clean. The rear brakes are loaded less in gentle driving, and on an electric or hybrid car even less, because the electric motor takes over much of the braking. That gives rust on the rear discs more time to eat in.

No, not on surface rust or a narrow rust lip. According to the RDW APK manual, it is a fail when less than 50 percent of the braking surface per side is smooth and rust-free, or when there is a risk of the disc breaking. In case of doubt, the inspector records an advisory. Too little braking force on the brake tester can also lead to a fail.

Yes, damp air and rain are what cause surface rust, so a car parked outside in Rotterdam will show it often. That by itself is no problem, because the pads scrub it off as soon as you brake. It matters more how often the car is driven and whether the brakes are used. A car that stands for weeks, or an electric car that rarely uses its mechanical brakes, is where rust gets the chance to eat in.

• ANWB: Remschijven: roest, levensduur en vervangen per as (brake discs, in Dutch)
• RDW: APK manual for passenger cars, components and operation of the braking system (article 5.2.31, brake discs, in Dutch)
• Ferodo: Brake Judder: thickness variation, hot spots and blue spots
• ANWB: Welk onderhoud heeft een elektrische auto nodig? (maintenance of an electric car, rust on the mechanical brakes, in Dutch)
• ADAC: E-Autos gewinnen bis zu 40 Prozent Energie zurück (press release 15 March 2024, rust risk and 22 percent, in German)
• Tesla: Model 3 Owner’s Manual (EU), Maintenance Service Intervals