Corrosion is a global challenge that design engineers must address to reduce failures throughout the equipment life cycle. The electrochemical reaction that costs billions of dollars, both in direct and indirect costs, may lead to catastrophic incidents if not properly managed or prevented.
Direct expenses are incurred while repairing and replacing the corroded systems, while indirect expenses arise from physical and environmental impacts of the corrosion-related failures.
Effective corrosion control requires that designers integrate measures at an early stage in the design phase. By working with materials scientists, reliability and maintenance specialists, the designer should design reliable and cost-effective parts or equipment that provide the intended service throughout the service life.
An understanding of the causes and types of corrosion is important when deciding on suitable materials and finishes that match a specific environment.
Design Decisions in Corrosion Control
Corrosion is a complex problem that arises from multiple competing factors that vary with an application. The actual rates, location and extent of corrosion are dictated by the environment, equipment design, metallurgy and geometry.
A lack of accurate data and changing environmental factors make it difficult to get accurate corrosion rates, especially on new equipment and in new environments. Laboratory tests are sometimes used to test and predict corrosion processes in a certain environment. (To learn more about corrosion testing, read Corrosion Assessment: 8 Corrosion Tests That Help Engineers Mitigate Corrosion.)
The materials, corrosion protection methods and application methods are design elements that should be thoroughly evaluated, and applied effectively and economically. The expected life, risk of corrosion and whether to integrate control measures in the fabrication or to be carried out through preventive maintenance are things that must be balanced to ensure that the design is reliable and economical throughout the equipment service life.
The following factors must be considered for effective corrosion control:
- Causes of corrosion
- Types of corrosion
- Materials used
- Design details
Causes of Corrosion
Being an electrochemical reaction, corrosion will occur whenever there is the presence of:
- An anode and cathode, such as with two dissimilar metals. This may also occur due to a potential difference created in the two concentration regions of a differential electrolyte.
- A metallic conductor between the two electrodes
- An electrolyte such as water
Eliminating the causes may be restricted in practice, because of the function and feasibility. Avoiding dissimilar metal contact may be impossible because of cost, weight and functional issues. However, surface treatments, plating, coating, painting and sealing can minimize the corrosion potential. Water, on the other hand, cannot be avoided, but can be controlled using drain holes, sealants, drain paths and corrosion-inhibiting compounds.
The usage environment, presence of corrosion acceleration factors, and design-related corrosive micro-environments should all be considered at an early stage.
Common corrosive environments may contain one or a combination of the following:
- Carbon dioxide
- Hydrogen sulfide
Factors that increase the rate of corrosion include:
- Chlorides such as saltwater
- Solids, dirt and debris
- Wear and abrasion
Severe environments require upfront attention and adequate steps taken to ensure the sustainability of the equipment in its service life. Design costs increase as the environment becomes more severe.
Types of Corrosion
Different forms of corrosion exist; the type and severity depends on several factors, including the materials, environment and application. The designer should be aware of the corrosion type likely to occur in a particular environment based on the materials and processes, and hence integrate effective control measures.
Common types of corrosion include:
- Selective leaching
- Concentration cell
- Stress cracking
- Flow-assisted erosion
- Stray current
- Hydrogen embrittlement
- Fungus growth
The material selection is critical in minimizing corrosion risks, and suitable materials should be based on application, environmental analysis, cost and available corrosion protection methods. Other factors include weight, physical and chemical characteristics of the equipment, and the potential interactions between dissimilar materials. (For more about selecting the best materials, see What New Materials Science Studies Suggest About Corrosion Control in the Future.)
A material may be in its passive or reactive state depending on the environment. Understanding the usage environment is thus important in determining if a passive metal will remain in the desired state without becoming active throughout its service life.
Examples of passive states in which the metal has natural resistance to corrosion include stainless steel and aluminum in the air, and iron in nitric acid. On the other hand, aluminum becomes active and will corrode when in stagnant water, while iron will corrode in atmospheric and marine environment conditions.
Corrosion control measures should be present at all stages, such as design, transportation, installation, handling and integration. Material surfaces should be protected and considerations made for joining, welding, gaps, adjacent materials, drainage and other contributing factors. The designer should include provisions for in-service cleaning, inspections, coating and corrosion protection compounds reapplication.
Proper material selection, design and fabrication details, finishing, plating, drainage, sealing, galvanic coupling of materials, the use of corrosion-inhibiting compounds and suitable in-service corrosion control programs are all critical factors for corrosion control:
- Material – Passive metals should be used whenever possible, and added protection such as coatings applied when there is a risk of the metal becoming reactive.
- Sealants – When liquids cannot be avoided, they should be drained properly and sealants used to avoid liquid ingress.
- Finishing – A suitable surface finishing and coating is an effective means of controlling corrosion.
- Drainage – Prevent fluids from becoming trapped in crevices.
- Galvanic coupling of materials – Designs should avoid coupling different materials unless required by weight, economic and functional considerations. When dissimilar metal coupling is necessary, appropriate finishing, sealing and other corrosion control methods should be applied.
- Use of corrosion-inhibiting compounds – In addition to finishes, sealants and drainage provisions, corrosion-inhibiting compounds offer added protection and should be reapplied periodically depending on the particular environment. (Inhibitors are discussed in How to Select the Best Volatile Corrosion Inhibitor (VCI) for Your Application.)
- Eliminating other environmental contributing factors – This is achieved by maintaining the pH, controlling gas leakages and breakout, removing oxygen, lowering chloride levels, and controlling bacteria and other contaminants. Care should be taken to maintain safe and uniformly distributed temperatures. Physical factors such as velocities and pressures should also be kept within acceptable levels without affecting the efficiency of the system.
- Access for maintenance – Easy access to all parts of the equipment or system should be provided for regular corrosion inspections and routine maintenance.
- Effective corrosion control programs – The designer should lay down an effective in-service maintenance procedure and inspection. A comprehensive control program maximizes corrosion protection and minimizes the downtime associated with corrosion-related maintenance and failures.
- Environmental issues – The design should ensure that materials used for the equipment, for corrosion protection and those from the processes, have minimal or zero toxic and hazardous emissions into the environment.
Understanding the factors contributing to the various types of corrosion in any specific environment is important in designing equipment with suitable corrosion protection that will last throughout its life cycle. Material selection, the environment and control measures must be addressed early in the design phase to establish an optimum and reliable solution.