Corrosion Protection of P/M Parts by Hot Dipping

Sintered ferrous powder metal parts and components exhibit inferior corrosion resistance compared to wrought materials because of the presence of open porosity. Zinc coatings applied by electrodeposition or electroless depostion offer corrosion protection. In the present study, hot dipping using molten zinc has been examined as an alternative approach to metal coating for corrossion protection of sintered iron-base parts. The quality and corrosion protection offered by the coating was evaluated as a function of part shape and microstructure and from salt spray and potentiodynamic polarization tests, and the-determination of tolerances. Hot dipping was shown to result in thicker coatings than in electrodepostion with improved corrosion protection; the zinc coating seals open pores and covers areas which are difficult to reach with other deposition techiques. The change in tolerance is higher for hot dipping than for electrodeposition. It is concluded that hot dipping is a rapid, low cost coating process for corrosion protection that does not require pretreatment.