An In-Depth Look at Materials Selection
Materials selection involves complex procedures that an require understanding of failure susceptibilities and corrosion control methods, as well as cooperation among the different engineering disciplines.
Materials selection is a major corrosion control method in engineering design. It plays a role in the effective estimation of project costs, construction scheduling and safe operations.
The fundamental concepts of materials selection pertain include:
- Material Types: This covers the broad range of available materials that can be selected.
- Material Properties: This pertains to examining the mechanical, physical and corrosion-resistance properties that fulfill requirements for specific applications/uses.
- Material Economics: This pertains to the costs of ownership, capital, maintenance and operation for various materials.
An Overview of Common Materials
There are many engineering materials in the industry. Here we'll take a look at the properties of some of the most common ones.
Carbon steel refers to a family of iron-based metals with less than 2% carbon. Carbon steels are the most used materials thanks to their ease of fabrication, low cost, availability and high strength. Carbon steels are used in a wide range of applications, from structural components to high-pressure equipment.
Carbon steels alloyed with minor additions of chromium, molybdenum, vanadium and niobium are considered low alloy steels. The alloying elements form carbides in the microstructure, increasing the materials' strength and reliability at high temperatures, such as in power plant boilers.
At subzero temperatures, carbon steels have less toughness, so nickel is added to maintain toughness. With higher nickel amounts, the range for lower temperatures increases, especially for liquid gasses applications to prevent brittle fractures in liquefied natural gas (LNG) and liquefied petroleum gas (LPG) equipment.
Cast iron is an iron-based metal, with more than 2% carbon. It is cheap and does not need additional treatment to reduce the carbon content. However, it is brittle and has limited weldability. It is used for non-critical parts in water pumps and pipes, and has corrosion resistance similar to that of carbon steels. Grey cast iron and nodular/ductile cast iron are the most common types of cast iron.
Stainless Steel (SS)
Stainless steel is a steel type with chromium of at least 11%, in combination with nickel, molybdenum, and nitrogen. Stainless steels are reliably used in corrosive environments in a variety of energy, chemical and petrochemical industries. There are several types of stainless steel, depending on the alloying contents and microstructures.
Ferritic Stainless Steels
Ferritic stainless steels are the most economical types and have very low nickel content. They are susceptible to sensitization and difficult to weld, and are thus used for unwelded components. Despite their moderate corrosion resistance, ferritic stainless steels are resistant to chloride-assisted stress corrosion cracking (SCC).
Martensitic Stainless Steels
Martensitic stainless steels have low nickel content, with a range of uses similar to that of ferritic stainless steels. They have high strength and there are many pipelines fabricated from martensitic stainless steels. Their high hardness make them preferred to use against erosion.
Austenitic Stainless Steels
Austenitic stainless steels are the most commonly used stainless steels in industry. With higher nickel, they have good corrosion resistance and are easily fabricated. There are different grades of austenitic stainless steels to select from depending on the environment's corrosivity. Some lower alloyed grades have low resistance to chloride SCC. Austenitic stainless steels are extensively used in cryogenic applications.
Duplex Stainless Steels
Duplex stainless steels have dual microstructures, ferritic and austenitic, with combined properties. They have high strength and very good corrosion resistance, especially against chloride-assisted SCC, making them good for use in offshore applications. However, duplex stainless steels inherit the lower strength of the ferritic phase.
Precipitation Hardened Stainless Steels
Precipitation hardened stainless steels have high strength and toughness, from austenitic, semi austenitic, or martensitic microstructures, depending on the heat treatment. Although they have good weldability, their applications are limited, such as for high-performance springs.
An alloy combines different metals, allowing for different properties. Here are some of the most common alloys and their key characteristics.
Nickel alloys have exceptional corrosion resistance, but they are expensive because of their high nickel content, which is usually added in combination with other alloying elements like molybdenum. There are many types of nickel alloys that are designated by brand name, such as Hastelloy, Incoloy, Monel, etc, These names pertain to expired patents, but the naming legacy remains. Nickel alloys are resistant to chloride-assisted SCC and sulfide-assisted SCC.
Aluminum alloys have good strength-to-weight ratio and good corrosion resistance. They are, however, susceptible to galvanic corrosion when in contact with iron-based metals. As aluminum has a low melting temperature, the application of aluminum alloys is limited to low-temperature uses. Because of their excellent thermal performance and ductility retention, aluminum alloys are used in cryogenic applications, such as heat exchangers in the LNG industry.
Copper alloys are commonly used in seawater environments and heat exchangers. They have high resistance to biofouling, especially microfouling, and stagnant conditions that can cause localized corrosion, as well as corrosion related to excessive flow. Some copper alloys are susceptible to SCC in environments containing ammonia.
Titanium alloys have good strength-to-weight ratio and high corrosion resistance in high chloride environments. They are, however, expensive and difficult to fabricate. Unalloyed titanium is commonly used in industry and it is more economical than other titanium grades.
Because of their brittle nature - and despite their high corrosion resistance - ceramics are not used in pressure-containing equipment. Ceramics are used in high-temperature equipment and parts, such as refractories, and for protection against wear. Ceramics for refractories are generally composed of a combination of oxides of aluminum, magnesium and silicon. For wear resistance, ceramics are composed of carbides or nitrides.
Polymers are used as thermoplastics, thermosets and elastomers. Most polymers are very corrosion resistant, even more so than many corrosion resistant alloys. Polymers, however, have low strength, low melting points and limited UV resistance. In corrosive, high-pressure applications, polymers can be used for corrosion-resistant liners on metal surfaces. HDPE, PVC and PTFE are examples of thermoplastics, but despite being in the same group, they have different properties and applications. Thermosets are not usually used as solid materials, but rather as coatings or in a matrix of a composite. Elastomers have unique mechanical properties and are used for sealing components in many pressurized applications.
Composites are a combination of two or more materials to achieve both properties of both and therefore optimal characteristics for a certain application. Most common composites in industry combine glass fiber reinforced in polymer matrix, and steel reinforced in concrete matrix. Fiber gives high strength and matrix gives toughness, distributing stress uniformly. Reinforcement fiber could be made of glass, metal, carbon and other polymers such as aramid. E-glass is a commonly used fiber, and there are other types of glasses suitable for different applications, such as C-glass for chemical resistance or R-glass for fatigue resistance. Thermoset polymer is a commonly used composite matrix that is easy to manufacture and has good mechanical and corrosion-resistant properties. Thermoplastic polymer is not as commonly used as thermoset polymer, and it tends to be used for spoolable pipes for its high ductility.
Steel reinforced concrete is a vital structural material, and corrosion of steel occurs in corrosive environments such as those of seawater and wastewater. Coatings, cathodic protection, and use of stainless steels are used to mitigate corrosion.
Factors to Consider When It Comes to Materials Selection
In general, the major factors considered for materials include, but are not limited to:
- Mechanical properties
- Corrosion properties
- Total cost of ownership
- Ease of fabrication
For load-bearing applications, mechanical properties are checked to ensure structural integrity. Mechanical properties of engineering materials vary highly, depending on microstructures, chemical composition, and manufacturing processes. The varieties are standardized internationally or locally by governing bodies, such as ASTM, ISO or JIS. Basic mechanical properties include, but are not limited to:
- Tensile strength
- Impact energy
- Fracture toughness
- Other factors depending on the specific application
The properties are commonly obtained from destructive testing of samples from manufacturing processes, and the results are stated in material certificates. Specialized mechanical testing, however, is carried out beforehand to avoid disputes between purchasers and manufacturers.
Materials degrade over time by environment-induced reactions called corrosion. Environmental corrosivity varies depending on the nature of the environment, selected materials and application. Materials must be selected properly to ensure reliability and safe operation during the application lifetime.
Corrosion could be induced from the atmosphere, soil, water, chemicals, oil and gas, and microorganisms, and by different corrodants causing different types of corrosion. Materials selection for the purpose of mitigating corrosion generally uses the following:
- Thermodynamic corrosion diagrams/charts
- Corrosion models
- Chemical compatibility tables
- Laboratory testing (potentiodynamic polarization, immersion, electrochemical impedance spectroscopy, stress corrosion cracking)
- Technology licensor recommendations
- Operational feedback
Thermodynamic Corrosion Diagrams/Charts
Corrosion diagrams/charts are based on data collection from laboratory testing and/or field performance. The data collected are based on parameters studied in laboratory testing or field testing.
Corrosion models are used to predict corrosion rates based on empirical formulae. The models are used in the form of proprietary software programs or in excel sheets with input parameters entered. There are models for CO2 corrosion, oxygen corrosion and crack growth rate. The accuracy of models can vary, as not all corrosion parameters can be comprised in the models, which are empirically generated from experimentally obtained data.
Chemical Compatibility Tables
Chemical compatibility tables or charts are qualitative tools used to determine compatibility between the environmental conditions and the selected materials for an application. For some situations, additional investigations and testing are required to better determine the compatibility of a selected material.
Laboratory testing is carried out for cases for which analytical data are needed to evaluate the performance of selected materials in simulated environmental conditions. The environmental conditions, laboratory setups and experimental conditions are arranged in a laboratory environment standardized to ensure the obtained data represent the parameters used for effective and suitable materials selection.
Technology Licensor Recommendations
Some processing technologies are proprietary, and the know-how arrangements are owned by licensors. Materials selected by licensors often represent the minimum requirements for warranty purposes.
Operational feedback from actual experiences are valuable as they offer real-time, field-based data. Historical data collection (operational parameters changes, materials certificates, Welding Procedure Specification (WPS)/ Procedure Qualification Records (PQR), inspection reports, failure analysis reports, etc.) are crucial to guarantee suitable materials selection.
Assessing the Cost of Materials Selection Decisions
Cost is of major consideration for materials selection, and the balance of capital expenditures (CAPEX) and operating expenses (OPEX) is crucial. Materials and corrosion engineers should select materials that satisfy the considerations of budget and costs as well as performance. The significance of CAPEX lies not only in the bulk materials selected, but also in the fabrication processes and product forms. The impact of OPEX lies in the repair and replacement of corroded materials, and the corrosion control, corrosion monitoring and corrosion inhibitor injection. Using carbon steel might be of low CAPEX, but of high OPEX, and using stainless steel impacts each in the opposite way.
Materials availability also plays a crucial role, especially regarding project schedule. Use of proprietary materials, for example, might take longer than usual in delivery because of limitations related to the suppliers, low quantities, or stocks that could require minimum quantity orders, in situations that could increase cost.
Difficult-to-fabricate materials might significantly impact the project schedule if not planned early, especially when used in large quantities. For example, materials that need post-weld heat treatment (PWHT) might require thousands of weldments in oil and gas facilities, as well as thousands of man hours to accommodate PWHT requirements. Difficult-to-weld materials might slow the welding speeds and increase repair rates.
Materials selection involves complex procedures, which require understanding of failure susceptibilities and corrosion control methods, as well as cooperation among the different engineering disciplines.