What is and when to use Cladding
- Posted by: arvengtraining
- Category: Welding
Often the material that can resist corrosion from the environment of a particular application may not have adequate structural strength. Or because of its low strength it may require use of uneconomically thick sections. An economical solution consists of providing a layer of corrosion-resistant material of suitable thickness on the contact surfaces of the equipment, made from an economical and structurally strong material such as carbon steel. This method of combining two layers of different materials is called Cladding or Lining.
While the term Lining is of a non-generic nature and covers different and types of materials, the term Cladding is used when the corrosion-resistant layer provided is metallic and well-bonded to the surface. Therefore, for steel fabricated equipment such as pressure vessels and shell and tube heat exchangers the term Cladding is broadly used.
It is important to bear in mind that the structural strength is provided only by the base material. In addition to corrosion, the cladding must withstand the prevailing temperature, stresses that may be present, and any abrasion as well. Cladding offers an attractive economic alternative by providing a relatively thin corrosion barrier of a costly corrosion-resistant metal or alloy, with an inexpensive structurally strong base material able to withstand all loadings.
A corrosion-resistant metal or alloy layer may be fixed to the base metal by various means. Cladding is a method that consists of joining two sheets, the base material (resisting all loads), and the corrosion-resistant material welded or fused by means of different procedures. Another method is the Weld Overlay, consists of welding (manually or automatically) a corrosion-resistant metal to the base metal. In both cases the end result is the same, an economical strong base material withstanding the loadings plus a thin layer of a corrosion-resistant metal acting as a protective layer. These two methods described can be used alone or combined.
A good thumb rule for considering using cladding is the following. For vessels requiring a thickness of 75-85 mm (or more) to withstand the operating conditions, the construction of the whole vessel from the corrosion-resistant alloy may be prohibitively costly. A 3 mm layer of corrosion-resistant alloy clad on a 75-85 mm steel plate could reduce the cost substantially, providing the same corrosion resistance. However, cladding is not always the best technical-economical choice. The fabrication of cladded vessels is highly complex and expensive. Moreover, thermal cycling normally creates problems at weld point of sleeves and nozzles. Additionally, cracks or corrosion can occur at the welds where adjacent sheets are joined in clad-metal construction.
The economics of clad versus corrosion-resistant solid base materials construction depends on the unit cost of the specific material, its relative fabrication costs, and the complexity of the equipment. There are no general rules for when to choose one alternative or the other, each case should be evaluated independently.
If you want to know more:
ASME VIII | Design of Pressure Vessels
TEMA | Design of Shell & Tube Heat Exchangers
Piping Systems in Industrial Plants: I