Flexible Metal Hose Corrosion Resistance

Flexible Metal Hose Corrosion Resistance

General

Flexible metal elements are basically suitable for the transport of critical fluids if a sufficient resistance is ensured against all corrosive media that may occur during the entire lifetime.

The flexibility of the corrugated elements like bellows of corrugated hoses generally require their wall thickness to be considerably smaller than that of all other parts of the system in which they are installed.

As therefore increasing the wall thickness to prevent damages caused by corrosion is not reasonable,it becomes essential to select a suitable material for the flexible elements which is sufficiently resistant.

Special attention must be paid to all possible kinds of corrosion ,especially pitting corrosion,intercrystalline corrosion,crevice corrosion,and stress corrosion cracking.

This leads to the fact that in many cases at least the ply of the flexible element that is exposed to the corrosive fluid has to be chosen of a material with even higher corrosion resistance than those of the system parts.

Types of corrosion

According to EN ISO 8044,corrosion is the “physicochemical interaction between a metal and its environment that results in changes in the properties of the metal,and which may lead to significant impairment of the function of the metal,the environment,or the technital system,of which these form a part.This interaction is often of an electrochemical nature”.

Different types of corrosion may occur,depending on the material and on the corrosion conditions.The most important corrosion types of ferrous and non-ferrous metals are described below.

Uniform corrosion

A general corrosion proceeding at almost the same rate over the whole surface.The loss in weight which occurs is generally specified either in g/m2h or as the reduction in the wall thickness in mm/year.

Pitting corrosion

A locally limited corrosion attack that may occur under certain conditions,called pitting corrosion on account of its appearance. It is caused by the effects of chlorine,bromine and iodine ions, especially when they are present in hydrous solutions.

Intergranular corrosion

Intergranular corrosion is a local,selective type of corrosion which primarily affects the grain boundaries.It is caused by deposits in the material structure,which lead to a reduction in the corrosion resistance in the regions close to the grain boundaries.In stainless steels this type of corrosion can advance up to the point where the grain composition is dissolved.

Stress corrosion cracking

This type of corrosion is observed most frequently in austenitic materials,subjected to tensile stresses and exposed to a corrosive agent.The most important agents are alkaline solutions and those containing chloride.

Crevice corrosion

Crevice corrosion is a localized,seldom encountered form of corrosion found in crevices which are the result of the design or of deposits.This corrosion type is caused by the lack of oxygen in the crevices,oxygen being essential in passive materials to preserve the passive layer.

Dezincing

A type of corrosion which occurs primarily in copper-zinc alloys with more than 20% zinc. During the corrosion process the copper is separated from the brass,usually in the form of basic salts above the point of corrosion. The dezincing can be either of the surface type or locally restricted,and can also be found deeper inside.

Contact corrosion

A corrosion type which may result from a combination of different materials.Galvanic potential series are used to assess the risk of contact corrosion,e.g. in seawater.Metals which are close together on the potential series are mutually compatible;The anodic metal corrodes increasingly in line with the distance between two metals.