Effect of Heat Treatment

7.3.3 Effect of Heat Treatment

A carbon steel quenched from high temperatures has a structure called martens- ite — a supersaturated solution of carbon in iron — a single metastable phase with carbon in solid solution in interstitial positions of the body - centered tetragonal lattice of iron atoms. Random distribution of carbon atoms accompanied by electronic interaction of carbon atoms with neighboring iron atoms limits their effectiveness as cathodes of local - action cells; consequently, in dilute acid the

STEEL REINFORCEMENTS IN CONCRETE

corrosion rate of martensite is relatively low. Interstitial carbon reacts in large part with acid to form a hydrocarbon gas mixture (accounting for the odor of pickled steel) and some residual amorphous carbon, which is observed as a black smut on the steel surface.

Heating martensite to low temperatures (below 727 ° C) and then air - cooling (called tempering) results in the formation of tempered martensite, which con- sists of α - iron and many dispersed particles of cementite (i.e., iron carbide, Fe 3 C). This two - phase structure of α - iron and cementite sets up galvanic cells that accelerate the corrosion reaction. The amount of fi nely divided cementite that forms depends on the tempering time and temperature; for a 2 - h heat treatment of 0.95% C steel, the maximum cementite forms at about 400 ° C (300 ° C for 0.07% C steel). After tempering at this temperature, cementite, acting as cathode, offers maximum peripheral surface adjoining ferrite, and galvanic action during subsequent exposure to an aqueous environment is at a maximum. Above this temperature, cementite coalesces to larger particle size, and the subsequent cor- rosion rate is lower. The particles of cementite are now large enough to resist complete dissolution in acid and can be detected in the residue of corrosion products. At the same time, there is a corresponding decrease in hydrocarbon gas formation.

On slowly cooling a carbon steel from the austenite (face - centered cubic lattice) region, above 727 ° C, cementite, in part, assumes a lamellar shape, forming

a structure called pearlite . This structure again corrodes at a comparatively low rate because of the relatively massive form of cementite formed by decomposi- tion of austenite compared with smaller - size cementite particles resulting from decomposition of martensite. Corrosion rate increases as the size of iron carbide particles decreases. Pearlitic structures corrode faster than spheroidized ones, and steels containing fi ne pearlite corrode more rapidly than those with coarse pearlite [42] . The importance of both the amount of cementite acting as cathode and its state of subdivision supports the electrochemical mechanism of corrosion. The rate of corrosion is cathodically controlled and, hence, depends on hydrogen overpotential and interfacial area of cathodic sites.

In practice, an effect of heat treatment on corrosion is seldom observed, because oxygen diffusion controls the rate in the usual environments. However, in handling acid oil - well brines, marked localized corrosion is sometimes found near welds or “ up - set ” ends of steel oil - well tubing. This observed increase in corrosion encircling a limited inner region of the tube is called ring - worm corro- sion . It is caused by heat treatment incidental to joining and fabrication tech- niques, and it can be minimized by a fi nal heat treatment of the tubing or by addition of inhibitors to the brine [58] .