Understanding Carbon Steel Corrosion
Simply put, corrosion is the gradual destruction of a material under triggering environment. It is a natural phenomenon where an original material decomposes to its elements or end up forming another compound. If left unattended, corrosion can have huge economic effects, complete equipment failure and sometimes loss of lives. A study conducted in the past revealed the total direct costs associated with metallic corrosion at a staggering $276 billion on an annualized basis in U.S. alone.
The low cost and mechanical properties carbon steel offers makes it the most widely used construction material in various industries such as Oil & gas. However, the iron content in the carbon steel is typically more than 98% that makes it susceptible to corrosion. Using stainless steel is an alternative but alloying of iron on a large scale is not economically feasible. The employment of coatings or paintings could prevent the corrosion of iron, however, the limited life and technical difficulties of these measures are to be considered. How the world of corrosion eats away the industrial materials is a topic of immense interest and attention. Because corrosion has many aspects, it is usually useful to categorize the corrosion types.
Atmospheric Corrosion
Atmospheric corrosion stems out from the exposure of materials to the atmosphere that could be classified as being, rural, industrial, or marine in nature. This corrosion type is common and corrodes the carbon steel via an electrolytic process. A thin and invisible film of an electrolyte solution is usually formed on the surface under the atmospheric exposure and humidity levels above a critical value.
The industrial city environment is usually enriched with the oxides of Sulfur or Nitrogen, particulate matter and Chlorides that are highly corrosive. For instance, under humid conditions, sulfur dioxide being highly soluble in water makes a thin surface film onto the surface of carbon steel that is highly capable of electrolytic activity. Through a series of chemical reactions, sulfuric acid formed eats away the iron gradually. One of the most significant component of the marine environment is the chloride ion and its corrosion mechanism can be explained using an electrolytic phenomenon.
To study the effects of air pollution onto the iron samples, a study was conducted, where iron samples were subjected to a variable city environment for one year. These samples were observed to corrode easily when the air pollution and humidity were steep (winter time having higher concentrations of SO2 and NO2). The rate of corrosion was relatively lower in the summer/spring time.
Aqueous Corrosion
Numerous applications require carbon steel pipelines or vessels for services demanding wet environment such as water transportation and this water exposure leads to deterioration of the material through aqueous corrosion. The water in application has many characteristics such as its acidity or alkalinity (also known as pH), dissolved oxygen and ionic content, flow rate and temperature that influences the rate of corrosion. For example, in a test, dropping the pH of distilled water exposed to carbon steel increases the rate of corrosion. The drop in pH from 4.5 to 3.7 raised the rate of corrosion by over 8 times.
The diffusion of dissolved oxygen to the surface of carbon steel through water or surface film (diffusion barriers) is the governing corrosion mechanism in freshwater. Therefore, the increase in oxygen concentration, the relative velocity of freshwater or temperature (to a limit) is bound to increase the rate of corrosion. The seawater applications exposes materials to a huge diaspora of chemical, physical, or biological factors and like freshwaters, the corrosion rate depends upon the supply of oxygen. The seawater is characterized by five different corrosive zones namely, atmospheric, splash, tidal, submerged, and seabed-embedded and each zone has different corrosion output to carbon steel.
Soil Corrosion
Corrosion of carbon steel structures buried in the ground is affected by factors like soil resistivity, pH, soil moisture, oxygen availability and microbial activity. From the studies conducted in the past, it is wise to conclude that no single factor controls the overall corrosion rate, although general rules can be formulated. For instance, the soils exhibiting lower resistivity are prone to be more corrosive suggesting the use of resistivity as a rough index for soil corrosivity. Soils are not very corrosive environment and therefore, corrosion is usually modest. However, localized corrosion, if resulted, may increase the rate of penetration by a magnitude.
Underground pipelines are usually used to transfer fluid over long distances. The routing of line through different regional soils enhances the possibility of the formation of macro-galvanic cells (localized corrosion). This develop potential differences between different parts of the material, resulting in severe pitting. The damage from macro-galvanic corrosion is magnanimous in case the buried pipe comes in contact with a foreign metallic structure with no isolation between the two.
Stress Corrosion Cracking (SCC)
SCC starts with the development of a crack under the influence of tensile stresses and requisite environmental conditions. The tensile stresses could be either directly applied stresses during application or in the form of residual stresses induced during fabrication through welding, heat treatment, machining, and so on. These residual stresses, if not relieved, can promote the probability of SCC. The cracks developed may have an intergranular (cracks running along the grain boundaries) or transgranular (cracks run through the individual grains) propagation and sometimes, combination of both. Carbon steel is prone to SCC in a wide range of environments. Few common environmental factors being the presence of ions like chloride, ionic concentration, dissolved oxygen, pH, and temperature, etc.
The frequency of failures though SCC appears to be increasing globally especially in the last few decades. For instance, SCC is probably the biggest challenge to transfer fuel grade ethanol through the carbon steel structures. A characterized study conducted revealed the effects of ethanol chemistry upon SCC behaviour of carbon steel sample. Slow strain rate testing (SSRT) was used to evaluate the susceptibility of carbon steel to SCC. Results suggested chloride ions strongly affecting the crack initiation and growth with higher concentrations leading to higher crack density. The other factors affecting the SCC were oxygen levels and water concentration.
