Copper in Steels

Copper in Steels  Copper (Cu) provides interesting alloying advantages to steel and steel welds. Cu (atomic no. 29 and atomic weight 63.54) has a density of 8.96 gm/cc. The melting point of Cu is 1083 deg C while the boiling point is 2570 deg C. Cu is normally added to steel to increase corrosion resistance. Experimentation with the addition of Cu to structural steels for its ability to impart good atmospheric corrosion resistance properties began in the year 1916 in the USA. The first commercial use of Cu bearing steel as atmospheric corrosion resistant steel was during the period 1933 to 1935. Cu is beneficial to atmospheric corrosion resistance when present in amounts exceeding 0.20 %. The atmospheric corrosion resistance steels are also called weathering steels. Addition of Cu in steelmaking Cu is added to steel in the form of pigs, refined ingots or as copper and nonferrous alloy scrap. Any Cu already present in the steel scrap charge is carried over into the liquid steel with little or no loss. Scrap electric motors containing usually 10-15 % Cu are often used as a source of Cu during steelmaking. Cu can be added either in the steelmaking furnace or in the teeming ladle. Both techniques of Cu addition give very high recoveries. Losses, if any, are due to the mechanical reasons. When addition of Cu is made in the steelmaking furnace, it should be done before the end of decarburization. Effects of copper on steels The ability of Cu bearing steel to withstand atmospheric corrosion is not the major reason which provides the impetus for the development of Cu bearing steels today. Until recently, it was not fully recognized that along with the improved corrosion resistance, the precipitation of Cu can make a significant...

Weathering Steels

Weathering Steels All low alloy steels have a tendency to rust in the presence of moisture and air. This rust is a porous oxide layer which can hold moisture and oxygen and promote further corrosion. The rate of rust formation depends on the access of oxygen, moisture and atmospheric contaminants to the metal surface. As the rusting process progresses, the rust layer forms a barrier to the ingress of oxygen, moisture and contaminants, and the rate of rusting slows down. The rust layers formed on most conventional structural steels detach from the metal surface after a certain time and in the process exposes the surface once again to rusting and thus commencing the corrosion cycle again. The rate of rust formation progresses as a series of incremental curves approximating to a straight line. The slope of this straight depends on the aggressiveness of the environment. Weathering steels are weather resistant steels which work by controlling the rate at which oxygen in the atmosphere can react with the surface of the metal. These steels are high strength low alloy steels which can provide corrosion protection without additional coating. Increase in alloying elements, mainly copper, provides an arresting mechanism to atmospheric corrosion in the steel itself. The alloying elements in the steel produce a stable and durable rust layer that adheres to the base metal. This rust ‘patina’ develops under conditions of alternate wetting and drying to produce a protective barrier, which impedes further access of oxygen and moisture. This patina acts as a skin to protect the steel substrate. Section loss on the order of 100 mils (2.54 mm) may be expected before the patina sets up, but this is negligible to the structural performance. Comparison of rate of rusting in low alloy steel and...

Corrosion of steel reinforcement bars in concrete Apr07

Corrosion of steel reinforcement bars in concrete...

Corrosion of steel reinforcement bars in concrete Reinforced concrete is a composite material comprising steel reinforcement bars embedded in a concrete mass.  Reinforcement bars carries the bulk of the tensile load and imparts a degree of cracking resistance to the concrete which itself is compressively loaded. The corrosion of steel reinforcement bars in concrete is a big universal problem. The damage which happens from corrosion may due to a large extent reduce the serviceability and structural integrity of reinforced concrete. Concrete provides stable long term corrosion protection to steel reinforcement bars because of passivation of the surface of steel by the highly alkaline property of the concrete. As long as this passivity is maintained, the corrosion of the reinforcement bars will not take place. For this the concrete should be sufficiently dense and impermeable so as not to allow transport of chlorides, carbon di oxide and oxygen. The passivation of steel reinforcement bars is deteriorates greatly due to chlorides even at high pH values. The carbon oxide in solution neutralizes the calcium hydro oxide saturated pore water (carbonation process) and lowers the pH value below the pH value at which the steel can remain passivated. Oxygen also plays a vital role in the passivation process. Whenever the reinforcement bar embedded in steel corrodes, the corrosion products increase its volume. All forms of iron oxide and hydro oxide have specific volumes greater than that of steel. The volume of the corrosion products vary by a factor which can be even more than 5 as indicated in Fig. 1. The expansive forces generated by the steel corrosion leads to tensile cracking and rust staining of the concrete. This in turn causes reduction in the serviceability and structural integrity of concrete besides affecting its aesthetics. Once the...