Influence of microalloying on the corrosion resistance of steel in saturated calcium hydroxide
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I.
INTRODUCTION
U N T I L a few years ago, most reinforcing bars supplied in the hot-roiling condition were made from carbon-manganese steel containing about 0.3 pct C and 0.8 pct Mn. These steels had a yield strength on the order of 250 N/mm2 and gave elongation of about 20 to 25 pct.tlJ In the past 2 decades, there has been an increasing demand for higher strength reinforcing bars to permit lighter construction to be made with thinner sections, reducing substantially the tonnage requirements and transportation cost.t2-5~ In many national specifications, a minimum yield strength of 410 N/nlIn2[1] or 490 N/nln1216,7,s] is required. This high yield strength was first achieved by raising the carbon, manganese, and/or silicon contents. However, the increasing demand of reinforcing bars with higher allowable stress, excellent bendability, and weldability made these types of steels with higher carbon and high silicon content unsuitable. In recent years, successful improvement in the mechanical properties of low-carbon steels has been achieved by microadditions of strong carbide-forming elements such as niobium, vanadium, or titanium. Such microalloyed steels have higher strength levels as a result of grain refinement of ferrite. The resulting fine-grained structure is often further strengthened by fine precipitates of particular alloy carbide. By using a microalloying technique, weldable high strength reinforcing steel bars were successfully produced. However, using a thinner section of these high strength steels in structural concrete necessitates a reasonable corrosion resistance. Corrosion of steel in concrete is a well-known problem with large economic consequences. Many reviews have been written concerning this problem, yet it is still unclear how the concrete protects steel. The problem of corrosion of reinforcing steel in concrete has attracted the attention of many authorstg,~~ over the last 2 decades. While much research has been conducted on this subject, the mechanisms that explain protection of steel in concrete are still
M.M. HEGAZY, Lecturer of Chemistry, is with the Dept. of Chemistry, Helwan University, 1143 Cairo, Egypt. M.M. EISSA, Assistant Professor, is with the Steel Metallurgy and Ferroalloys Department, CMRDI, 11421 Cairo, Egypt. Manuscript submitted September 9, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
subject to dispute. The corrosion behavior of carbon steel in Ca(OH): solutions has become of increasing practical interestY q The corrosion of steel and other metals in concrete lends itself to examination by various electrochemical techniques.[~2] Attention has been concentrated upon linear polarization resistancev3j and a.c. impedance, t14] By contrast, transient techniques have received only limited attention despite their convenience and simplicityY5~ With the objective of producing high strength steel bars with a minimum yield strength of 600 N/mm2, two steel compositions are produced using vanadium or titanium as a microalloying element. The corrosion resistance of these high str
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