Zinc-aluminium-magnesium (ZM) coatings are increasingly used by carmakers to protect steel parts from corrosion. Advanced ZM coatings, such as Zagnelis® from ArcelorMittal, are allowing OEMs to use thinner protection while providing new opportunities for weight savings. But with the rapid development of automotive steels and coatings, how can OEMs be sure the coating will stand up to long-term use? ArcelorMittal is going the extra mile by testing its automotive coatings on trucks which criss-cross Sweden.
Substituting pure zinc coatings with advanced solutions such as Zagnelis® will lead to significant weight savings on a vehicle. For example, replacing the pure-zinc coatings Z100 or Z140 with a zinc-magnesium-aluminium coating such as Zagnelis® ZM70 or ZM100 can reduce coating weight by between 30 and 50 percent. However, the level of corrosion protection is the same and part stamping is made easier.
Corrosion testing on trucks
“With Zagnelis® Surface available for both exposed and non-exposed parts, the weight saving can be quite interesting for carmakers,” notes Laurence Dosdat from ArcelorMittal R&D. “To help convince OEMs that thinner Zagnelis® coatings could offer the same protection, ArcelorMittal decided to test our advanced coatings on trucks and get some real data from the field.”
ArcelorMittal sends samples of Zagnelis® and standard zinc coatings to the Swedish Corrosion Institute which handles the testing procedure. The samples are mounted on a frame which is then attached to the underside of a truck. The samples travel the same route from Sweden’s capital Stockholm to the country’s second-largest city Gothenburg and back for at least one year.
“While the route is always the same, the climatic conditions and actual number of kilometres covered can vary,” says Laurence Dosdat. “Sensors on the trucks record air temperature and humidity, while corrosivity is evaluated on pure and steel coupons. That enables us to measure and compare the effect of these parameters on Zagnelis®.”
Aggressive environment accelerates testing
“On-truck testing is quite aggressive compared to site-based testing in coastal locations, mainly due to the elevated levels of pollutants and salts present on the roads,” continues Laurence Dosdat. “But that means we get the results more quickly. And we can reach a Cx corrosivity level on zinc. This is the highest category described in the industry standard for atmospheric corrosion (NF EN ISO 9224).”
The results of ArcelorMittal’s on-truck testing have shown that zinc-magnesium-aluminium coatings provide significantly better protection against perforating corrosion in areas without paint (most critical areas). No red rust is observed after a year of testing, unlike the zinc-galvanised unpainted reference samples. Weight loss measurements for Zagnelis® Protect indicate the corrosion rate of this advanced coating is at least two-times slower than for a standard zinc-based coating.
ArcelorMittal’s on-truck testing strategy helps simulate the real-part testing that OEMs carry out themselves as Laurence Dosdat explains: “The German automotive industry has developed new corrosion tests which recognise the new steels and coatings on the market. But many OEMs still carry out their own tests of these new materials by making real parts. The parts are added to a prototype vehicle and then driven on a range of test tracks for up to six months to simulate actual driving conditions. This vehicle is also tested using test benches and corrosion chambers.
“The on-truck testing ArcelorMittal is carrying out simulates real-world conditions and gives OEMs more confidence in the coating,” continues Laurence Dosdat. “And it encourages them to test our new coatings on their own prototype vehicles.”
Experts in corrosion and testing
Some OEMs require new products to be approved through laboratory-based accelerated corrosion tests. To meet these requirements, ArcelorMittal has large testing facilities and an active test programme at static natural sites which are monitored by us or by recognised institutes.
“We can test very large parts in our walk-in corrosion chamber, or subject the same part to different tests at the same time in different chambers,” says Laurence Dosdat. “We’ve just finished a six-year project in partnership with various OEMs and the French Corrosion Institute’s land-based test site in Brest. These tests proved that our Zagnelis® coating is an efficient solution in various corrosive environments around the world including marine, urban, rural, and tropical areas. Our expertise in corrosion and testing is helping OEMs to create durable and sustainable mobility solutions for everyone.”
The walk-in accelerated corrosion chamber at ArcelorMittal Maizières can be used to test large automotive parts.
For more information, see also ArcelorMittal's online product catalogue.
ArcelorMittal Europe – Flat Products is constantly commercialising advanced steel grades for automotive applications to meet the changing needs of the global mobility industry. ArcelorMittal also researches and tests new steel coatings which will protect and extend the life of our automotive offer. Two of the latest automotive coatings released by ArcelorMittal are Jetgal® and Zagnelis®.
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