The corrosion resistance of a substrate can be improved by metallurgically bonding to the susceptible core alloy a surface layer of a metal or an alloy with good corrosion resistance. The cladding is selected not only to have good corrosion resistance but also to be anodic to the core alloy by about 80 to 100 mV. Thus if the cladding becomes damaged by scratches, or if the core alloy is exposed at drilled fastener holes, the cladding will provide cathodic protection by corroding sacrificially.
Cladding is prevalently applied at the mill stage by the manufacturers of sheet, plate or tubing. Cladding by pressing, rolling or extrusion can produce a coating in which the thickness and distribution can be controlled over wide ranges and the coatings produced free of porosity. Although there is almost no practical limits to the thickness of coatings that can be produced by cladding, the application of the process is limited to simple shaped articles that do not require much subsequent mechanical deformation.
Among the principal uses are aluminum cladding in the aircraft industry, lead and cadmium sheathing for cables, lead-sheathed sheets for architectural applications and composite extruded tubes for heat exchangers. The thickness of the cladding is usually between 2% and 5% of the total sheet or plate thickness, and since the cladding is usually a softer and lower strength alloy, the presence of the cladding can lower the fatigue strength and abrasion resistance of the product. In the case of thick plate where substantial amounts of material may be removed from one side by machining so that the cladding becomes a larger fraction of the total thickness, the decrease in strength of the product may be substantial.
A clad finish being soft in nature is subject to damage during manufacturing and while in service. Caution must be exercised while polishing or cleaning, since it is sensitive to harsh chemicals and abrasive materials.
Cladding and Weld Overlaying
Compared to carbon and alloy steels, all corrosion resistant alloys are expensive. In many cases, corrosion resistance is required only on the surface of the material and carbon or alloy steel can be clad with a more corrosion resistant alloy. Cladding can save up to 80% of the cost of using solid alloy. Cladding of carbon or low alloy steel can be accomplished in several ways including roll bonding, explosive bonding, weld overlaying and “wallpapering”. Clad materials are widely used in the chemical process, offshore oil production, oil refining and electric power generation industries. The use of clad steel is not new. Corrosion resistant alloy clad steel has been available for over 40 years. Almost any corrosion resistant stainless steel or nickel alloy can be bonded to steel. The steel can be clad on both sides or on one side only. The hot roll bonding process is used to produce over 90% of clad plate products.
Approximately 60,000 tons of hot roll bonded corrosion resistant alloy clad steel plate is produced annually. In this process, specially cleaned plates of steel and the corrosion resistant alloy are placed together and hot rolled. A metallurgical bond is formed between the steel and the corrosion resistant alloy through the combination of heating and deformation during rolling. In most cases, multiple “sandwiches” of alternating steel and corrosion resistant alloy plates are hot rolled in one operation for economy. The thickness of the cladding is normally in the 1.5 to 5 mm range although thicker cladding is possible. Total hot roll bonded plate thicknesses of from 6 mm to 20 cm are available in widths of up to 4 meters and lengths of up to 20 meters.
Clad plate can be formed and fabricated using conventional techniques. The steel is welded using ordinary welding materials and practices. The clad material is welded using high alloy filler metal. The techniques for welding clad steel are well developed. The explosive bonding process utilizes the high-energy impulse from an explosive charge to drive together the surfaces of the metals to be bonded. The plastic deformation of the metals during the high -energy collision forms a metallurgical bond between them. Explosive bonding can join almost any two (or more) metals. Explosive bonding can be used to fabricate plate, pipe, fittings, and other shapes. Cladding thicknesses for explosive bonding usually range from 1.5 mm to 2.5 cm.
Cladding with corrosion resistant alloys thinner than 1.5 mm is difficult. In some cases, explosive bonding is followed by hot rolling to improve the bond between the steel and corrosion resistant alloy. Weld overlaying is commonly used to clad the surfaces of fabricated steel structures. The actual weld overlay process used depends on many factors including access, welding position, the alloy applied, and economics. In some alloy combinations, dilution of the weld overlay material by the steel requires that more than one weld pass is required. Post weld heat treating to temper the backing steel may be required in some cases. Wallpapering, also known as sheet lining, was originally developed in the 1920’s to fabricate stainless steel clad steel vessels for the chemical process industry. In wallpapering, thin sheets of corrosion resistant alloy are edge welded to the carbon steel structure.
In some cases where larger wallpapering sheets are used, plug welds at intervals on the wallpapering sheet are also required. Wallpapering can be a very economical way to provide excellent corrosion resistance for steel structures. Both stainless steels and the more corrosion resistant nickel alloys can be economically applied to steel by wallpapering. Wallpapering has also been widely used to line interiors of stacks and ducting for flue gas desulfurization units in fossil fuel power plants. One major benefit of wallpapering and weld overlaying is that they can be used to repair or modify existing steel structures. (reference: James F. Jenkins. P.E.)
See also: Cladding, Electroplating, Pack cementation, Electroless plating, Vapor deposition, Hot dip galvanizing, Thermal spraying, Zinc coatings