Corrosion Evaluation of Chromized Steel Utilized in Automotive Exhaust Applications

Abstract

Experiments were conducted to determine the suitability of a chromized steel for use in automotive exhaust applications. Due to government regulations leading to higher temperatures and a more corrosive environment within the automotive exhaust system, Cr-lean alloys such as Type 409 no longer suffice. The high cost of increasing alloying elements to reduce the corrosion susceptibility of exhaust components has led to exhaust manufactures moving toward a sacrificially protected aluminized stainless steel (Type 409Al). Yet, costs remain high due to the stainless steel substrate. Arcanum Alloys have designed a process in which an IF steel coil is chromized using a Cr-rich slurry, creating a thin but corrosion resistant layer. This chromized layer drastically increases the corrosion resistance, without affecting the formability of the interstitial-free (IF) steel substrate and remaining cost-effective. The localized corrosion resistance of the chromized IF steel (XHOM) was measured against current generation ferritic stainless steels in a simulated interior (exhaust gas condensate) and exterior (NaCl (aq)) automotive exhaust environment. Electrochemical polarization measurements along with atmospheric corrosion tests were conducted to characterize and compare the localized corrosion susceptibility of XHOM and benchmark ferritic stainless steels. The specific tests include the following: I. Potentiodynamic polarization curves in NaCl (aq), measuring the corrosion potential (Ecorr), critical current density (icrit) and breakdown potential (Eb). II. Double loop electrochemical potentiokinetic reactivation (DLEPR) testing measuring the ratio of the activation critical current density (ia) and the reactivation critical current density (ir). III. Salt-fog testing (ASTM B117) (external) and exhaust gas condensate exposure testing (internal), measuring the mass loss, pitting density, maximum pit depth and corrosion rate. Although Type 409 and Type 439 exhibited evidence of sensitization in the mill annealed condition, all materials exhibited a resistance to further sensitization during heat treatment, indicating sensitization will not occur during service. The electrochemical polarization curves in the NaCl (aq) resulted in XHOM yielding the highest breakdown potential, yet XHOM also exhibited the highest corrosion rate during the salt fog (ASTM B117) exposure. The latter is due to exposure of the XHOM cut edge where only the plan surface was exposed during the electrochemical polarization measurements. A galvanic couple exists between the chromized coating (cathode) and steel substrate (anode) leading to rapid corrosion of the substrate when exposed. When the cut edge of XHOM is masked, the corrosion rate drops drastically, performing comparably to the highly ferritic stainless steels. During the salt fog (ASTM B117) exposure, pitting of XHOM and Type 409 was caused by cut edge corrosion leading to corrosion product migrating down the panel surfaces and initiating under deposit pitting. A singular pit was observed on the XHOM surface, which led to delamination of the coating surrounding the pit, caused by the galvanic couple at the coating substrate/interface once the substrate was penetrated. The overall corrosion resistance ranking of the materials in the external environment incorporating corrosion rate and pit depth is as follows: Type 436 ≈ XHOM Masked Edges ≈ Type 439 > Type 409Al > Type 409 > XHOM Edges Exposed. Strain was also found to have an effect on the localized corrosion susceptibility of XHOM in NaCl (aq), unlike Type 409, which exhibited no change. The influence of the drain hole manufacturing method (punching and drilling) on the corrosion susceptibility of XHOM and Type 409 was also measured. The punching method caused a smearing effect of the chromized coating, which served to partially cover and protect the cut edge. The main corrosion mechanism that occurred within the external environment is cut edge corrosion, which led to under deposit pitting. Heat treatment of samples prior to testing in the internal exhaust environment led to an intermetallic phase change within the aluminized coating on Type 409Al, drastically reducing the corrosion resistance of the material. An as-received aluminized Type 409 (Type 409Al-A) sample was tested in exhaust condensate exposure conditions to measure the difference in corrosion rate. XHOM with the cut edges exposed exhibited a corrosion rate comparable to Type 409 and heat treated aluminized Type 409, which is promising as XHOM already has an advantage in cost and formability. The overall corrosion resistance ranking of the materials exposed in the internal exhaust environment incorporating corrosion rate and pit depth is as follows: Type 409Al-A > Type 436 > Type 439 > Type 409 ≈ XHOM Edges Exposed ≈Type 409Al-H (heat-treated).