ABSTRACT

This study has evaluated the galvanic coupling corrosion risk between connection bolt materials with ASTM A1010 structural steel, which has recently emerged for the construction of more corrosion-resistant bridges than weathering steel bridges. Use of A1010 steels containing 10.5% Cr is intended to extend the service life of steel bridges without frequent maintenance requirements caused by corrosion, particularly in regions under severe chloride exposures such as due to heavy use of de-icing salts and in marine environments. The greater corrosion resistance of A1010 steel with a more positive corrosion potential does, however, impose a risk of galvanic corrosion with the connection bolts in direct electrical contact. Here, the galvanic corrosion between A1010 steel and galvanized ASTM A325 Type I bolt was compared to that between weathering steel and galvanized ASTM A325 Type I bolt, as well as that between A1010 steel and ASTM A320 B8 class 2 and A193 B6 stainless steel bolts. A comprehensive experimental investigation measured galvanic coupling current and potential for samples in cells of aerated salt solution, and the galvanic corrosion risk ranking was validated by visual examinations of bolted steel plates exposed to a salt spray testing chamber. The galvanic corrosion risk of using B8 class 2 bolts with A1010 steel was found to be much lower than using galvanized A325 bolts, but B6 bolt material itself experienced severe crevice and pitting corrosion in both simulated salt solution and salt spray testing.

INTRODUCTION

Steel bridges represent more than 30% of the total number of highway bridges in the United States 1 and Canada. Highway bridges currently utilize two types of steel, carbon based grades and weathering steel grades. They are predominantly constructed from steel in accordance with the composition and properties adopted by the American Society for Testing and Materials (ASTM) such as ASTM A709 50W, HPS 50W and HPS 70W, A588 2 Carbon based steels are prone to rapid rates of atmospheric corrosion, and require supplementary protective coating systems to sustain the design service life of steel bridges. The steel coating systems have a much shorter service life than steel bridges; e.g. 20 years versus 100 years, and thus repeated applications and maintenance are needed. Weathering steels were used to attempt to avoid this problem as they form a protective rust (corrosion product) layer, patina, which can be dense and adherent to steel, during a long-term exposure to the atmosphere. This protective patina can reduce corrosion rate to less than 0.01 mm/year 3. For example, weathering steel has been used as bridge construction material by the Ministry of Transportation of Ontario (MTO) Canada since 1968, and it has been used exclusively since then for structural steel components in highway bridges in Ontario4.

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