Highways Agency investigations have detected intergranular corrosion and cracks propagating through the microscopic structure of the bearings' hardened stainless steel. A side effect of intergranular corrosion is the liberation of hydrogen. Highly hardened materials such as the failed rollers can be susceptible to stress corrosion and hydrogen embrittlement, says Dobson. The hardening properties of the stainless steel surface are generally imparted by quenching and then tempering to increase the toughness.
When designing with stainless steel there is a straight trade-off between mechanical properties and corrosion resistance, a leading bearing manufacturer claims.
The stronger the material, the lower its resistance to corrosion.
The next stage of the investigation will seek out the cause of corrosion, before the responsibility of the failure can be determined.