Fundamental research on corrosion resistance of precoated steel sheets for automobiles.

Abstract

SynopsisCorrosion behaviors of Precoated sheets for automobiles were studied basically. Weight loss of unpainted specimens, paint adhesion and perforation resistance were measured. Corrosion of unpainted sheet in a salty environment is cathodically controlled. Cathodic reactions are retarded effectively by corrosion products with a low electrical conductivity, such as Zn(OH)2 and ZnC12.4Zn(OH)2.Al, Mg, Ni, Co or Mn alloyed Zn coated sheets exhibit a good performance because of the formation of above mentioned corrosion products. Paint adhesion depends greatly on the nature of the underlying phosphate layer. During cathodic elect rodeposition of Zn coated sheet part of the phosphate film dissolves and gets defects because of high alkalinity. On the other hand, Zn-Fe and Zn-Ni alloy coated sheets have the defects repairing effect, and therefore show a better paint adhesion than Zn plated sheet. Perforation corrosion is caused by the formation of differential aeration cells in several car components. Perforation resistance was evaluated by the tests in a combined corrosive environment using jointed panel specimens. Heavil 'Zn coated sheets and V primer coated sheets demonstrated a high resistance.

Influence of Zinc Ions on Initial Stage of Localized Corrosion of Zn and Zn–Al Alloy Coated Steels with Photon Rupture Method

“…The corrosion protection of the coated layers is ascribed to cathodic protection by a galvanic reaction between the coated layer and substrate, 1,2) and the formation of stable and compact corrosion products, which have high corrosion resistance. Composition of corrosion products formed on the steels caused by atmospheric corrosion has been investigated in various exposure conditions.…”

Initial Stage of Localized Corrosion in Artificial Pits Formed with Photon Rupture on 55mass%Al–Zn Coated Steels

“…The corrosion protection of the coated layers are ascribed to cathodic protection by a galvanic reaction between the coated layer and substrate, [1][2][3] and the formation of stable and compact corrosion products which have high corrosion resistance. The composition of corrosion products formed on steels caused by atmospheric corrosion has been investigated in a range of exposure conditions.…”

The new look of sheet steels.

“…It has been suggested that the corrosion products from dezincification become trapped in these microcracks, preventing the transfer of the corrodant to the base steel. 17,21,22) It has also been proposed that the corrosion product that forms in zinc-nickel alloys is zinc hydroxide, Zn(OH)2, which in itself is effective in suppressing the oxygen reduction rate. 22) Thus, although zinc-l3 % nickel electrodeposited alloy initially has a simple, single-phase structure, the corrosion protection mechanism is complex, involving the reduced activity of the coating, the gradual change of the coating from sacrificial to barrier protection and secondary barrier protection provided by the corrosion product, possibly by entrapment within microcracks in the coating that form during corrosion.…”

“…Thus as corrosion proceeds in NaCI solutions the coating gradually becomes richer in nickel and, hence, more electropositive. 17,21) An additional protection mechanism is that the residual stresses in the coating become strongly tensile during dezincification, leading to the formation of microcracks in the coating. It has been suggested that the corrosion products from dezincification become trapped in these microcracks, preventing the transfer of the corrodant to the base steel.…”