Effect of heat treatment on the corrosion of welded low-carbon steel in 0.3 M and 0.5 M of hydrochloric acid and sodium chloride environments at ambient temperature (25 oC) has been investigated. Arc welded low-carbon steel sample of known composition were subjected to the corrosion reagents for 21 days (504 hours). pH and weight loss values were taken at interval of 3 days. Thereafter, weight loss method was used to measure therate of corrosion attack on the heat treated samples at ambient temperature. Results obtained showed that at low concentration, the annealed sample exhibits better corrosion characteristic as compared to the normalized and quenched samples. However, at higher concentration the normalized sample exercised better service performance over the annealed and quenched samples. Thequenched sample was found to have relatively low corrosion performance over the annealed and normalized samples at both low and high concentrations of the media.
Carbon steels are the most important alloys used in petroleum and chemical industries since they account for over 98% of the construction materials. Among the most widely used of these alloys is low-carbon steel, its wide range of applications such as chemical, oil gas storage tanks and transportation pipelines is due to its moderate strength, good weldability and formability [
Pipelines deterioration as a result of corrosion has come to be accepted worldwide as an unavoidable fact of life. 2241 major pipeline accidents were reported [
The present degradation of infrastructures particularly in the salt and acid environments has continued to generate a lot of worries to the researchers in this noble area in the view to procure lasting solution to the problem. Metallurgical control of corrosion includes inhibition, coating and heat treatments [
Heat treatment is a method of altering the physical and sometimes chemical properties of materials. The most recent study of cost of materials lost to corrosion in the United Kingdom carried out by the government committee on corrosion and protection was put at a staggering rate of £350 billion per annum. Corrosion, like taxes and death is inevitable especially in the chemical and petroleum industries [
The materials used for this work are low carbon steel in form of a plate of 10 mm thickness, multi-purpose electrode (gauge 10) and corrosion reagents.
The chemical analysis of the “as-received” low-carbon steel sample was conducted by optical emission spectrometry with an AR430 metal analyzer. The chemical composition is shown in
Specimen of dimension 10 × 10 × 10 mm was cut from the original sample with hack saw, polished with 240, 320, 400, 600, 1000 and 1200 grits of emery papers respectively and etched with 2% nital solution. Microscopy studies were then carried out on the prepared sample using optical microscope at magnification of 400×.
Twenty (20) samples of 10 × 10 × 10 mm dimension were cut from the remaining steel plate using hack saw, the samples were then divided into four (4) groups A, B, C and D with each group having five samples. Samples in group A serves as control while samples in groups B, C and D were welded using electric arc welding with general purpose electrode (gauge 10).
All the welded samples were stress relieved to remove internal stresses imposed on them during welding [
Samples C were annealed by heating them to 920˚C soaked for 30 minutes and cooled in the furnace to ambient temperature. Samples D were heated to 920˚C soaked for 30 minutes and cooled in water at ambient temperature. All the heat treatment samples were prepared for metallographic analysis; they were consecutively polished with emery papers of grades 240, 320, 400, 600, 1000 and 1200 grits to remove scales resulting from heat treat processes.
Corrosive media of Conc. HCl and Conc. NaCl were prepared with 0.3 M and 0.5 M. Thereafter, the welded and the non-welded heat treatment samples were exposed to the media for 21 days (504 hours) for corrosion attacks. Weight loss—a measure of difference between the original mass of the sample before immersion (M1) and the mass of the same sample after exposure (M2) was taken at interval of 3 days and corrosion rate in mil per year is calculated using the recommended ASTM relation Corrosion rate = W/A·(T/365)
where:
W = Weight loss (gram);
A = Total area of exposure (cm2);
T = Exposure time in hours;
g/mm2/yr = gram per square mm per year (corrosion rate units).
pH was measured on daily basis for 21 days (504 hours) using Buffer tablets stabilized pH meter.
Corrosion action on the test samples were assessed by visual observation and corrosion rate measurement. Formation of corrosion products (light-green colour material) on the surfaces and the edges of the steel samples was observed to have occurred after 24 hours of exposure, but much earlier in samples attacked by 0.5 M and 0.3 M of Conc. HCl. Areas of attack on the samples by the environments were visually observed to be more for the quenched sample as compared to the normalized and annealed samples, and the oxidation products were observed to be uniformly laid on the samples’ surfaces and edges.
Optical microscopy study of the “as-received” nonwelded low carbon steel (