ABSTRACT

This paper presents a comparative evaluation of Al, Zn and A1-Zncoatings on carbon steel~ exposed to a coastal-marine atmosphere. It is a very aggressive atmosphere with high wind velocities (corrosion- erosion rate = 1.4 mm/year for ASTM 1029 steel). A metallized zinc coating with a pore sealer was also evaluated. 1S0 and ASTM standards were used for the evaluation.

After a two-year exposure the best performance was achieved by the Al and Al rich alloy (85% Al- 15%Zn) with some damage of the coatings. But the Zn metallized coating, with a pore sealer, did not show signs of damage.

INTRODUCTION

Carbon steels have excellent mechanical properties. Although easy, low-cost manufacture makes their use attractive, their resistance to atmospheric corrosion is very low in most environments and progressive deterioration of their structure leads to rust formation and consequent loss of some of their mechanical properties. In environments with little atmospheric pollution, atmospheric corrosion or rust formation is insignificant. However, expansion in the chemical~ oil, metallurgical and thermoelectric industries has increased atmospheric pollution notably, with generally greater aggressiveness on bare materials. So, because of their good performance in inclement weather, materials such as zinc, aluminum and alloys thereof are being used as protective coatings for carbon steels.

The protective efficacy of non-alloyed steels by means of metallic coatings depends on its capacity 1) to act as an isolating barrier between the surrounding atmosphere and the underlying metal i.e. on its thickness, uniformity, adherence, lack of porosity and ductility; 2) to provide electrolytic protection for the steel~ acting as a sacrificial anode should a cell or galvanic couple be formed. The latter situation could arise on cut edges or in areas where part of the metallic coating may accidentally be lost*.

In theory, a metallic coating applied to a metallic substrate creates a continuous barrier that completely isolates the base metal from the environment. Unfortunately, it is very difficult to achieve it in practice, since discontinuities such as pores, pitting and fissures are produced from the very moment of application. Besides, as coatings are prone to damage during transportation~ the galvanic action at the base of a pore or damaged area becomes an important factor when determining coating features2.

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