Introduction
Steel tubes may require surface treatment to improve corrosion resistance, prepare the surface for further processing or meet application requirements. The appropriate treatment depends on the base material, operating environment and intended use of the tube.
Steel tube surface treatment is not a single process, but different methods including passivation and phosphating will be selected for different purposes. Understanding the differences can help buyers to specify suitable tube finishes and avoid treating all protective coatings as the same.
What Is Passivation of Steel Tubes?
Passivation is a surface-treatment process, commonly used with suitable corrosion-resistant alloys, which assists in removing surface contamination (e.g. free iron) and in forming or restoring the protective passive surface associated with stainless steel.
Passivated steel tubes may be specified where it is necessary to control surface contamination and to support the corrosion resistance of the material.
How Does Passivation Work?
The tube surface is cleaned and treated by a suitable passivation procedure. The treatment chemistry and process conditions will depend upon the material and applicable specification. Rinsing and post-treatment handling may also form part of the procedure.
Passivation does not add a metallic coating to the tube, and it differs from plating which deposits another material onto the surface.
When Is Passivation Appropriate?
Passivation is commonly considered for stainless steel components where contamination of the surface could affect the corrosion resistance. The required procedure should be compatible with the alloy and the intended service environment.
Passivation will not make every steel grade corrosion-proof, and the base material, surface condition and operating environment remain important.
What Is Tube Phosphating?
Tube phosphating is a chemical conversion treatment in which a phosphate layer is formed on a suitable metal surface. The layer formed can support paint adhesion, provide a base for further coatings or provide a degree of corrosion protection as part of a specified system.
What Is Phosphating Used For?
Phosphating is often used where a tube or component will receive a subsequent coating, or requires a prepared surface for further manufacturing. Its suitability depends on the base metal, treatment process, coating system and environmental conditions.
Phosphate treatment is not the same as passivation or metallic plating, but produces a different layer and serves a different purpose.
What Are Zinc-Plated Steel Tubes?
Zinc plated steel tubes have a zinc coating applied to the steel surface by a plating process. Zinc can provide a protective barrier to the steel and, in suitable conditions, provide sacrificial protection to the underlying material.
How Does Zinc Plating Protect Steel?
Zinc separates the steel from the environment, and if the coating is damaged, it can provide sacrificial protection in appropriate conditions to the exposed exposed steel over a limited area.
The level of protection will depend upon the coating thickness, coverage, surface preparation, exposure and the requirements of the selected process.
Zinc plating should not be assumed to be appropriate for every temperature, chemical environment or application, and the coating specification must be compatible with the intended service conditions.
How Do Passivation, Phosphating and Zinc Plating Differ?
| Treatment | Basic function | Common consideration |
|---|---|---|
| Passivation | Helps remove surface contamination and support the passive surface of suitable alloys | Commonly specified for stainless steel |
| Phosphating | Forms a phosphate conversion layer | Often used as a base for paint or further coatings |
| Zinc plating | Deposits a zinc coating on steel | Used where a specified zinc coating is required for corrosion protection |
The treatments are not interchangeable, and a process should be selected according to the base material, required finish, environmental conditions and relevant specification.
How Should Buyers Select a Surface Treatment?
Before selecting a treatment, buyers should consider:
- The base material and grade.
- Exposure to moisture, chemicals and other environmental conditions.
- Required corrosion protection.
- Whether further painting or coating is to be planned.
- Surface cleanliness and finish requirements.
- Applicable standards, inspection methods and acceptance criteria.
The treatment should be specified alongside the tube dimensions and material requirements.
What Is Industrial Tube Finishing?
Industrial tube finishing includes the processes used to achieve the required surface condition after or during tube manufacturing. Depending upon the product, this can include cleaning, heat treatment, passivation, coating, straightening, cutting, inspection and protective packing.
Not every tube will require the same finishing sequence, and the process should be selected according to the base material, intended application and purchase specification.
How Does Sainest Tubes Address Surface Treatment?
Sainest Tubes Pvt. Ltd. lists passivated tubes amongst its product offerings and publishes information about manufacturing, inspection and protective finishing processes. The appropriate surface treatment should be matched to the tube material, required finish and intended application.
Conclusion
Passivation, phosphating and zinc plating serve different purposes in steel tube finishing. Passivation supports the protective surface of suitable corrosion-resistant alloys, phosphating provides a conversion layer and zinc plating deposits a metallic coating. Selecting the correct steel tube surface treatment requires consideration of material, service conditions and the relevant product specification.