5 Most Common Factors Affecting Corrosion
When metals are exposed to factors such as diffusion, conductivity, pH imbalance, electrochemical potential and certain types of ions, corrosion may take place more rapidly and through various mechanisms.
There are many factors essential to corrosion. By knowing what they are and dealing with them appropriately, you will be better equipped to prevent corrosion's harmful effects.
Here are five of the most common factors affecting corrosion:
In the majority of cases, the corrosion rates of metals are controlled by the diffusion of reactants to and from the metal surface.
Freshly exposed bare steel surfaces normally corrode at a greater rate than those covered with a compact layer of rust. The corrosion rate is majorly controlled by the diffusion of oxygen through water to the steel surface. In areas where oxygen diffusion is prevalent, corrosion happens at a faster rate. High-flow areas, such as in the vicinity of bell mouths, will tend to have higher corrosion rates because of the increased oxygen levels. Erosion is also a factor.
Areas covered by a thin, conducting moisture film normally corrode faster than areas under immersion. Therefore, the hullage space at the top of ballast tanks and at the top of double bottom tanks where air has become trapped corrode more quickly than deeply submerged areas where oxygen is less available.
For corrosion to take place, there must be a conductive medium between the two parts of the corrosion reaction. Corrosion will not occur in distilled water and the rate of corrosion is bound to increase as the conductivity increases due to the presence of more ions in the solution.
Steel's corrosion rate maximizes close to the normal ionic content of sea water. Fresh water corrodes steel to a lesser extent than brackish water, and seawater is normally the most corrosive to steel. (For more on corrosion in seawater, read: An Intro to Pipeline Corrosion in Seawater.)
pH is a measure of the acidity or alkalinity on a scale of one to 14. A pH of seven is neutral.
In neutral seawater, the pH is around seven and a half, indicating the hydrogen ions (i.e., the acid) and the hydroxyl ions (the alkali) are almost in balance. Under such conditions, the reaction that balances the iron dissolution is the reduction of dissolved oxygen to form hydroxyl ions.
If the environment becomes more acidic and the pH falls closer to one, there are more hydrogen ions than hydroxyl ions in the solution. The excess hydrogen ions can get involved in the balancing (cathodic) reaction, which results in the evolution of hydrogen gas. As both the hydrogen ions and the hydrogen gas diffuse very rapidly, the steel corrodes faster.
Under alkaline conditions, where there is an excess of hydroxyl ions and the pH levels tend towards 14, steel does not corrode and remains unaffected.
4. Electrochemical Potential
Every metal assumes a specific electrochemical potential when immersed in a conducting liquid. This potential is called the half-cell potential, as it can only be measured in comparison to another known reference potential produced by a reference electrode.
Common reference electrodes include the Saturated Calomel Electrode (SCE), silver/silver chloride and copper/copper sulfate. The potential a metal assumes in a solution can determine if, and how quickly, it will corrode. This potential can be changed by connecting the metal to a dissimilar metal, as in galvanic corrosion or by using sacrificial anodes, or by applying an external potential, as in active cathodic protection systems like those employed on the external hull. (For more on corrosion electrochemistry, read: Corrosion Electrochemistry: The 6 Electrochemical Reactions Involved in Corrosion.)
5. Type of Ions
Some types of ions present in seawater or in cargoes are more corrosive than others.
Chloride ions, for example, are usually the most destructive due to their effects on rust's protective properties. Chloride ions prevent the formation of more protective, densely packed oxides, most commonly in stainless steel, causing pitting corrosion where the passive chrome oxide layer gets attacked.
Sulfate and other sulfur-containing ions can also result in major problems. Sulfur-containing ions result in additional electron-generating reactions within rust, which subsequently forms a cyclic, self-regenerating process. This can produce intensive pitting on the inner bottoms of cargo tanks and other equipments in oil and product carriers. The sulfur can originate from both the inert gas system and from cargoes containing sulfur, such as sour crude oil.
Factors Affecting Atmospheric Corrosion
While the above factors pertain to all types of corrosion, almost all kinds of general corrosion take place within the atmosphere. Thus, it's equally important to understand factors affecting this specific corrosion subtype.
Let's dive into five of the most common factors affecting atmospheric corrosion:
1. Moisture, Dew and Condensation
Moisture, whether in the form of dew, rain or condensation, is a very significant factor when it comes to atmospheric corrosion. Although rain can wash away hazardous air pollutants in the atmosphere that have been deposited in exposed areas, such as in a marine environment, rain also collects in crevices and pockets. Rain can also hasten the corrosion process through constant wetness, especially in areas with galvanized bolts and steel parts or structures.
Moreover, condensation and dew are unwanted types of moisture when they are not washed away by recurrent rain, which could eliminate or dilute the contamination. Dew films that have become saturated with acid sulfates, sea salt and other acids could produce an aggressive electrolyte environment that promotes the occurrence of corrosion.
In humid, tropical areas, condensation can be found on surfaces. On such surfaces, the moisture can become stagnant, which turns into an alkaline reaction with metal or absorbs carbon dioxide to create a dilute acid.
Temperature affects atmospheric corrosion. Essentially, every 50-degree Fahrenheit (10 degree Celsius) increase in temperature can double corrosion activity.
Also, metallic objects are susceptible to temperature lag because of the heat capacity behind ambient temperature changes. When ambient temperature falls in the evening, the surfaces of metal objects or structures become warmer compared to the humid air that surrounds them. Thus, condensation will not begin until the dew peak has been attained.
When the temperature increases within the air in the environment, the temperature lag in these metals will turn them into condensers that maintain a moisture film on their surfaces. This increases the wetness period compared to the period when the ambient air is below the dew point. This also depends on the metal thickness, structure, air currents and sun radiation.
Cyclic temperature can result in severe metal corrosion in tropical areas, especially in unheated warehouses, objects in plastic bags, metal tools and more. (For more on corrosion protection in warehouses, read Temporary Corrosion Protection during Storage, Transportation and Handling.)
The dew point in the surrounding air is a sign of the evaporation and condensation equilibrium. Thus, it is highly recommended to sustain the temperature around 50 to 59 degrees Fahrenheit (10 to 15 degrees Celsius) above the dew point to guarantee corrosion will not occur due to surface condensation that may be colder compared to the ambient air.
3. Relative Humidity
Relative humidity is the water vapor quantity found in the atmosphere relative to the quantity of saturation at a certain temperature. It is typically expressed as a percentage.
Among the essential requirements in the process of atmospheric corrosion is the existence of an electrolyte in the form of a thin film that can develop on steel or metal surfaces that are exposed to critical humidity levels. Although the film is invisible, it can contain corrosive contaminants at high concentrations, particularly in situations where there is alternate drying and wetting.
The critical level of humidity depends on the material undergoing corrosion. It also depends on the product’s tendency to corrode, the absorption of moisture by surface deposits and the existence of pollutants. For instance, the critical level of humidity is at 60 percent in cases where the environment is without pollutants. With the presence of electrolytes in thin films, atmospheric corrosion carries on through the combined cathodic and anodic reactions. Anodic oxidation involves corrosion attack on metals.
Typically, marine environments and salt-rich aerosols have high levels of relative humidity. There are various studies that revealed an adsorbed water layer on zinc can have increased thickness along with an increase in relative humidity. This results in increased corrosion rates. (Learn more in Five Key Factors in Understanding the Role of Soluble Salts in Coatings Failures.)
4. Aerosol Particle Deposition
Aerosol particles' behavior in outdoor environments can be understood by the laws covering their movement, formation and capture. These can be found throughout the layers of the planetary boundary. The concentrations rely on several factors such as time, location, local sources, atmospheric conditions, wind velocity and altitude. There are also studies that show that aerosol capture, deposition and wind speed are related. These studies involve saline winds that have an excellent correlation between deposition rates of chloride with a particular wind speed threshold.
Aerosols that are a main factor in atmospheric corrosion can be produced by either chemical processes in the atmosphere or via ejection. Wind dust and sea spray are common aerosols. There are also secondary aerosols that are generated by condensing and reacting atmospheric gases, or through the conversion of gas into particles of cooling condensation. When aerosols remain suspended within the surrounding area, these can be removed, modified or destroyed. Aerosols will not stay in the surroundings indefinitely, as their average lifetime ranges from days to one week, but this still depends on the location and size of the particles.
Most aerosols are deposited near shorelines and these are typically huge particles that have a short lifespan and depend on gravitational forces. Aerosols are also influenced by gravity, wind resistance, solid surface impingement and dry-out droplets.
5. Presence of Pollutants
The existence of pollutants is a factor in atmospheric corrosion. For instance, sulfur dioxide, which is produced by fuel combustion that contains gasoline, diesel fuel, natural gas and sulfur, is considered one of the most harmful pollutants that can cause metal corrosion. (For more on how crude oil causes corrosion, read The 6 Corrosive Components That Can Be Found in Crude Oil.)
Other pollutants include nitrogen oxides, which are also combustion products. These can be found in vehicle fumes and can form reactions with UV light and moisture to develop new chemicals that can be carried as aerosols. A typical example is the haze during the summer, especially in huge cities where most of the haze is the result of a combination of nitric and sulfuric acid.
When metals are exposed to the aforementioned factors, corrosion may take place more rapidly and through various mechanisms.
Thus, understanding these factors could greatly help asset owners manage corrosion and its harmful effects.
Written by Raghvendra Gopal | Founder & Director, Materials Info Consultancy Private Limited
Raghvendra Gopal is a post graduate in Industrial Metallurgy from IIT Chennai, India. He holds a bachelor’s degree in Metallurgy from IIT Roorkee, India. Currently he is the owner of Materials Info Consultancy Private Limited in Pune, India, which is mainly involved in materials testing, failure analysis, new product development support, benchmarking and value engineering for the automotive, engineering and manufacturing sectors. Materials Info Consultancy works on corrosion related failures, investigation and simulations, and also works for developing a supplier base for various commodities including castings, forgings with compete inspection and testing as per end client requirements for domestic and international clients.
With over twenty five years of experience in the field of material selection for specific applications, failure analysis and light weight construction, working with engineering, automotive tier 1 and automotive OEMs, Raghvendra’s areas of interest include material selection and evaluation for specific applications, materials data generation, root cause failure analysis and selection of appropriate manufacturing processes.
Raghvendra was instrumental for a surface protection strategy and defining a corrosion warranty while working with an automotive OEM for various components, correlating corrosion life data obtained in the laboratory in comparison to field life.