Screenshot: Villaägarnas Riksförbund (YouTube)

A Swedish insurance company partnered with a Swedish homeowner’s organization to commission crash tests of a rusted Mazda6 wagon and a rusted Volkswagen Golf to see how badly the corrosion compromised the cars’ structural integrity. Here’s what the study found.

Swedish insurance company Folksam teamed up Villaägarnas Riksförbund, a Swedish organization that “promote[s] and protect[s] the interest of homeowners,” to commission the crash tests of two rusty cars at the Thatcham Research Centre in the United Kingdom.

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According to Villaägarnas’ press release, Folksam says the results indicate that one of the cars saw a 20 percent higher risk of death in an accident when compared with a rust-free, new model. But, while this may sound conclusive, the full study implies that there’s a lot of uncertainty associated with rust’s affect on the car’s performance.

The cars used in the test were a first generation Mazda6 (representing 2003 to 2008 model years) and a fifth generation VW Golf (representing 2004 to 2008 model years), both of which were put through NCAP frontal and side crash tests so the results could be compared to the official NCAP tests conducted when the cars were new.

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As shown in the video above, those tests included a 40 mph frontal offset crash test with a deformable barrier and a 31 mph side impact from a moving deformable barrier.

Screenshot: Villaägarnas Riksförbund (YouTube)

The rusty VW Golf didn’t see much of a change compared to its crash test rating when new, with its EuroNCAP score dropping only slightly from 33 points to 32. But the Mazda 6—a vehicle known to have rust issues—saw a drop from 26 to 18 points, bringing it down from a weak four star rating to a weak three, Villaägarnas Riksförbund says. The organization summarizes the rusty 6's crash results, saying:

...According to Folksam, for Mazda 6 this is 20 per cent higher risk of being killed in a real accident.

It is not certain that another used Mazda 6 with serious rust damage had behaved just like in our crash. However, depending on where the rust is sitting, other negative and uncontrolled crosstalk effects could occur, compared to when the car was new. Due to the rust, the car is not deformed as intended and the ability to catch collision is deteriorated.

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Thatcham Research (Villaagarna)

Okay, so a 20 percent higher risk of death in a real crash test; that’s pretty straightforward, right? Rust is very bad for a vehicle’s crash test performance, one might deduce. Well, reading through the details of the study suggests that the results of this test are highly inconclusive.

The lower front crash rails, shown above, are very rusty, however, according to researchers, the corrosion likely did not affect results in the frontal offset test, with the report stating:

From Figure 2 and 3 the extent of the rust is very severe; however, it does not appear to have greatly affected the structure of the lower chassis leg. The welds on the left image have not failed despite being surrounded in rust...This level of deformation of the front chassis leg is to be expected from the ODB impact, but without any original 2003 test images it is impossible to say with certainty if this vehicle reacted differently.

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Thatcham Research (Villaagarna)

The unibody rails, too, despite significant corrosion, may not have performed any differently than they would have without the rust, with the report writing:

The point of failure is the spot welds between the two sections. Once these spot welds failed, the floor section was able to greatly deform into the passenger compartment. It was noted in the original 2003 Euro NCAP Test that the footwell of the vehicle ruptured, and without original test images it is difficult to say what the extent of this deformation was.

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Thatcham Research (Villaagarna)

Even in the side impact, all the rust in the side sills may not have been such a huge detriment, with the study writing:

Figure 10 shows the deformation of the sill after the MDB impact. The footwell can be seen to have buckled a large amount in the impact, which most likely caused the additional compression of the ribs in the dummy. Again it is difficult to say whether the corrosion caused additional collapse of the sill and B-pillar without knowledge of the original 2003 impact.

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Thatcham Research (Villaagarna)

In the end, despite headlines like “Major deterioration of crash safety for Mazda 6" (translated from Swedish) in the Villaägarnas Riksförbund press release, the results—as with most things in life—appear to be rather nuanced.

Thatcham Research Centre’s conclusion section begins by saying:

Without the original footage and full inspection report it is difficult to say with certainty about the effect of the condition of the vehicle on it’s safety performance.

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But it does go on saying that, based on the modifiers shown in the above plot—particularly the ones related to the head bottoming out on the steering wheel, body shell integrity and footwell rupture—it appears that the rusty car did indeed perform worse than the Mazda 6 that was tested in 2003.

“It is the opinion of the author that the corrosion of the vehicles did negatively affect the safety performance of these tests,” the conclusion states, though the report finishes by admitting the results aren’t quite conclusive:

However, the extent of this negative affect is not as severe as expected. Without modifiers, the scores between the 2003 and 2018 tests are very comparable, and only after the application of modifiers do the results begin to differ..Without a much deeper analysis of the structure of the crashed vehicles, would you be able to confidently comment upon the affect of the corrosion onto the crash performance of the structure of the vehicle.

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So basically, rust is—as one might intuitively think—most likely bad for the crash structure of an automobile. How bad is it? We don’t really know based on this testing, but this may be just one more reason you should work to keep rust off your car.