Welded steel pipe corrosion control in applications

Types of corrosion:
There are two general classifications of corrosion that cover most of the specific forms: direct chemical attack and electrochemical attack. In both types of corrosion, the metal is converted into a metallic compound such as an oxide, hydroxide, or sulfate. The corrosion process always involves two simultaneous changes: the metal that is attacked or oxidized suffers what may be called anodic change, and the corrosive agent is reduced and may be considered as undergoing cathodic change.

In the modern times, much has been done to improve the corrosion resistance of welded steel pipes used in a variety of applications such as improvement in materials, surface treatment, insulation, and in particular, modern protective finishes. All of these have been aimed at reducing the overall maintenance effort, as well as improving reliability. In spite of these improvements, corrosion and its control is a very real problem that requires continuous preventive maintenance.

In the recent years, surface finishing is a broad range of industrial processes that alter the surface of a manufactured item to achieve a certain property. Black steel pipe is named for the black oxide substance formed on its surface, which is one popular member of steel pipe kingdom in the steel pipe market today. However, the black oxide substance has no capacity of anti corrosion and anti rust. In other words, it is necessary to take some measures to protect the black steel pipe from rusting in daily applications. Technically, paint is the most commonly used material to protect steel. Paint systems for steel structures have developed over the years to comply with industrial environmental legislation and in response to demands from bridge and building owners for improved durability performance. For cold rolled steel pipes, the protective paint systems usually consist of primer, undercoat(s) and finish coats. Each coating ‘layer’ in any protective system has a specific function, and the different types are applied in a particular sequence of primer followed by intermediate / build coats in the shop, and finally the finish or top coat either in the shop or on site. Passivation is another important method to protect black steel pipe from corrosion in applications, which is widely applied to some structural steel pipes used outdoors.

Hot dipped galvanized steel pipe generally has the rational cost effective in the market today. Compared with other typical steel pipe coatings, such as specialized painting and powder coating, galvanization is much more labor-intensive, resulting in a higher initial cost for contractors. Besides, due to its durability and anti-corrosive properties, galvanized steel pipe can recycled and reused, which to some degree save a lot of money during the post maintenance work. Furthermore, for one thing, the galvanization process protects the steel from rusting damage that can occur during transportation, installation and service. The zinc layer on the surface of pipe can form a barrier protection for steel products to extend the service life in applications. For the other thing, this layer is also resistant to wear and scratch, which makes the steel look more attractive.