Part 1 of the seven-part series on material selection for chemical process equipment focuses on metals and alloys. Metals and alloys can be selected as materials ranging from larger structural systems to specialty small components for surface finishing process systems. Examples include:
- Structural frameworks, including hoist superstructures, catwalks, and tank supports, to extend beyond base floor elevation and also to suspend equipment from a structure above a process line.
- Material handling equipment, including hoists, shuttles, and carts.
- Process tanks and accessories (e.g., baffles, weirs, equipment mounts, and load saddles).
- Shells and internals of process equipment, including pumps, heat exchangers, and filters.
- Exhaust ventilation collection and air pollution control systems.
- Piping and valving systems for process fluids and process utility fluids.
- Electrical and Instrumentation and Control (I&C) cabinets and electrical conduit.
- Process accessories, including electrodes, mounts, fixtures, flight bars, and sensor probes.
- Grating, platforms, and ladders.
Different metals and alloys provide widely varying chemical resistance. There are also several orders of magnitude in raw metal or alloy cost, ranging from the more common metals and alloys to specialty high performance superalloys to precious metals. Various metals also range in important physical properties (e.g., density, yield strength, temperature range, formability, stiffness, weldability, electrical conductivity, ferromagnetism, and application-specific durability) that can impact material selection.
ASTM Structural Carbon Steel
These relatively low-cost steels are available in standard grades for structural shapes and plate and for structural pipe and tubing. Chemical resistance is limited for many chemical process applications. Liners and/or coating/paint systems are required for most applications of steel components in process plants for chemical exposure applications. Proper surface preparation using abrasive blasting and/or chemical pretreatments of steel components is critical with for successful application of liners and/or coating/paint systems. Primary applications for coating systems include superstructures, tanks support systems, and platforms. Primary applications for liners are process tanks.
Stainless Steel (SS) Alloys
Stainless steels (e.g., 304, 304L, 316, & 316L) are common in surface finishing applications. The alloy mix for 316SS alloys (~16% chromium (Cr), ~10% nickel (Ni), ~2% molybdenum (Mo) provides for superior chemical resistance, including many solutions containing chlorides and some acids, over 304SS alloys (~18% Cr, 8% Ni). However, there are applications where 304SS provides superior chemical resistance to 316SS. 304LSS and 316LSS are lower carbon alloys that are better for welding. Stainless steel alloys provide superior chemical resistance over carbon steel for a broad range of applications. Steel alloy prices vary with market conditions and alloy metal price variances. Relative prices also vary with form and quantity purchased. 316LSS plate is almost 30% higher in price* over 304LSS plate, and 304SS plate is roughly four times the price* of A36 grade carbon steel plate.
High Nickel Content Alloys
These higher cost alloys, such as Hastelloy C, provide superior corrosion resistance beyond SS alloys for some applications. However, some SS alloys provide superior corrosion resistance for some applications over high nickel alloys (e.g. – some phosphoric acid solutions). The nickel content of Hastelloy C276 is typically up to 56%. Hastelloy C276 is more than five times the price* of 316LSS and has approximately 10% higher density. The yield strength of C276 is similar to titanium.
Titanium (Ti) and Alloys
These metals and alloys range significantly in cost and provide superior chemical resistance for many applications. Titanium is lower density than 316LSS (~56%) and higher yield strength (~160%). Grade 2 (commercially pure) Ti has good weldability, strength, ductility, and formability and is the most common bar and sheet form for chemical process applications including tanks. While Grade 2 Ti is over four times the price* of 316LSS on a per weight basis, it can be only an approximate 50% price* premium over 316LSS for structural applications where Ti higher strength and lower weight factor into the equipment (e.g., tanks). Grade 5 Ti (aircraft grade) is the most common Ti alloy and accounts for ~50% of Titanium global use. Grade 7 Ti is similar in properties to Grade 2 but has interstitial palladium (Pd), making it the most corrosion resistant of all Ti alloys. Grade 7 Ti is almost five times the material price* of Grade 2 Ti. Some nitric acid applications are one example where stainless steels (304LSS in particular) provide superior corrosion resistance over pure Ti. Titanium provides superior chemical resistance for some chromic acid solutions, compared to 316LSS and Alloy 276.
Copper (Cu) and Alloys
In surface finishing, copper bussing (typically C11000 Electronic Tough Pitch – 99.9% Cu) is widely used above tanks for electrified processes. Copper is typically not recommended as a primary material for a range of acid and caustic solutions. A combination of good design for bussing routing, ventilation design, design to minimize dripping and splashing on bussing, and good process maintenance and housekeeping practices are needed to minimize copper bussing corrosion. In some cases, copper is plated or coated to control corrosion.
Many other metals and alloys are important for select process chemistry applications in surface finishing. Precious metals, such as gold (Au), palladium (Pd), and platinum (Pt), provide some of the highest levels of chemical resistance and are important metals for some process chemistry applications. These are extremely high in price and used only when essential. For some specialized applications, other noble metals, such as rhodium (Rh), ruthenium (Ru), rhenium (Re), and iridium (Ir), are used in pure form or in alloys (e.g., Ru is alloyed with Pt or Pd to increase wear resistance) for their differing physical and chemical resistance properties. Iridium is generally the most corrosion-resistant metal. Some other metals and applications are as follows:
- Tantalum (Ta), Zirconium (Zr), and Niobium (Nb) are significantly higher cost than titanium and provide superior chemical resistance in select applications (e.g., heat exchanger internal metals for aggressive process chemistries).
- A range of electrode metals are used for chemical processes application. In some cases, soluble metal anodes are used to make up solution chemistries. In other cases, metals are selected to provide more stable, inert electrodes. Electrode metals and alloys include copper, gold, iron, nickel, lead, platinum, silver, tin, titanium, and zinc and metal alloys, such as brass and tin-nickel.
- Life-cycle, cost-effective, insoluble anodes include more noble metal and metal oxide coatings comprised of Ir, Pt, Rh, Ru, and/or Ta over a lower-cost base metal (e.g., titanium).
Various metals and alloys are suitable for a wide range of surface finishing chemical process equipment applications. The process chemistry application range of lower cost metals can be extended with liners and with coating and paint systems (see Part 4). A diversity of plastics (see Part 2) and other materials, such as fiberglass (see Part 3), provide alternatives to metals for many surface finishing process chemistry applications. Life-cycle costs should be carefully evaluated for different material options for chemical process equipment.
Next week, watch for Part 2: Material Selection for Chemical Process Equipment – Plastics. Part 2 provides information on PVC and CPVC, PE (various densities), PP, PVDF, PTFE, and other plastics.
*- Early 2019 pricing. Metal and alloy prices vary with market conditions (e.g. – between late 2005 and late 2008 Ni metal price increased by more than 330% and then dropped to less than 20% of peak price.)
ITI is a global consulting, engineering, and design-build firm based in Burlington, Vermont. We specialize in manufacturing processes, water and wastewater treatment, recycling, and ventilation applications for the metal and surface finishing industry.
If you’ve received this email, you opted in to receive the Integrated Technologies, Inc. email newsletter at some point in the past. We only store your email address and name with the sole purpose of sending you our email newsletter on a quarterly basis.
You can change your mind at any time by clicking the unsubscribe link in the footer of this or any email you receive from us, or by contacting us at [email protected].