Effect of Heat Treatments on Corrosion of Welded Low-Carbon Steel in Acid and Salt Environments ()
1. Introduction
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 [1]. However, this material is susceptible to corrosion when used in chemical and sour crude oil environments.
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 [2] in the United Kingdom (UK) in the last 10 years and in the US alone the lost number on corrosion is approaching 350 million dollar per year [3]. In Nigeria, petroleum pipeline explosions occur regularly resulting in loss of lives and environmental pollution. There was a report of pipeline explosion in Idjerhe (Jesse) where hundreds of lives were lost [4]. A related incident in Adeje village near Warri, Delta State, where more than 250 Nigerians were fared died [5].
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 [6]. Over the years, inhibition and coatings have been the two major methods of corrosion control. Therefore, in this work attention is paid to heat treatment method of corrosion control.
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 [7]. Hence, if one considers the enormous amount of money spent in combating corrosion and the attendant grievous consequences of corrosion disasters then, this work is a right step in the right direction. Hence, studies were carried out on how the corrosion characteristics of welded low-carbon steel can be improved upon through quenching, normalizing and annealing heat treatment routes with a view to obtaining best route(s) for optimum service performance in acid and salt media at varied concentrations.
2. Materials and Method
2.1. Materials
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.
2.2. Method
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 Table 1.
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×.
2.3. Welding
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).
2.4. Heat Treatment
All the welded samples were stress relieved to remove internal stresses imposed on them during welding [8]. They were then heated to a temperature of 650˚C, soaked for 30 minutes and allowed to cool in the furnace to ambient temperature (25˚C). Samples B were then subjected to normalizing treatment by heating them to 920˚C soaked for 30 minutes and cooled in still air to ambient temperature.
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.
2.5. Preparation of Corrosive Media
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.
3. Results
3.1. Assessment of Corrosion Attack on the Test Samples
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.
3.2. Optical Microscopy Study
Optical microscopy study of the “as-received” nonwelded low carbon steel (Figure 1(a)) revealed predominantly pearlite phase in a matrix of ferrite. The dark areas correspond to pearlite and light areas depict the ferrite phase.
Figure 1(b) revealed the microstructure of the “as-received” welded stress relieved sample after corrosion attack. Visibly present are dark areas corresponding to