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Technical information about 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 or 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, intergranular corrosion, crevice corrosion, and stress corrosion cracking, SCC (see Types of corrosion).

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 have even higher corrosion resistance than those of the system parts it is connected to (see Resistance table).

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 technical 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/mh or as the reduction in the wall thickness in mm/year.

This type of corrosion includes the rust which commonly is found on normal steel caused by oxidation in the in the presence of water. Other types of eroding corrosion can be caused by liquids, such as acids, bases and salt solutions.

Stainless steels can only be affect by uniform corrosion under extremely unfavourable conditions (e.g. spatter rust or foreign rust).

Pitting corrosions

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.

This selective type of corrosion cannot be calculated, unlike surface corrosion, and can therefore only be kept under control by choosing an adequate resistant material.

The resistance of stainless steels to pitting corrosion increases in line with the molybdenum content in the chemical composition of the material.

The resistance of austenitic materials to pitting corrosion can approximately be compared by the so-called pitting index: PRE = Cr + 3,3Mo + 30N (“pitting resistance equivalent”, using the percentage of the mentioned components) whereas the higher values indicate a better resistance.

Sectional view (50-fold enlargement)
Plan view (50-fold enlargement)

Pitting corrosion on a cold strip made of austenitic steel

Intergranular corrosion (IGC)
Is a localized attack at and adjacent to grain boundaries, with relatively little corrosion of the grains caused by deposits in the material structure, which lead to a reduction in the corrosion resistance in the regions close to the grain boundaries. This type of corrosion may even lead to decay of the grain structure in stainless steels.

Intergranular corrosion (decay)
in austenitic material 1.4828

Sectional view (100-fold enlargement)

These deposit processes are d3ependent on temperature and time in CrNi alloys, whereby the critical temperature range is between 550 and 650°C and the period up to the onset of the deposit processes differs according to the type of steel. This must be taken into account, for example, when welding thick-walled parts with a high thermal capacity. These deposit-related changes in the structure can be reversed by means of solution annealing (1000 – 1050°C).

This type of corrosion can be avoided by using stainless steels with low carbon content (≤ 0,03% C) or containing stabilizing elements, such as titanium or ­niobium. For flexible elements, this may be material qualities like 1.4541, 1.4571 or 1.4306.

The resistance of materials to intergranular corrosion can be verified by a standardized test (Monypenny-Strauss test according to ISO 3651-2). Certificates to be delivered by the material supplier, proving resistant to IGC according to this test are therefore asked for in order and acceptance test specifications.

Stress corrosion cracking (SCC)
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.

The form of the cracks may be either transgranular or intergranular. Where­as the form only occurs at temperatures higher than 50°C (especially in solutions containing chloride), the intergranular form can be observed already at room temperature in austenitic materials in a neutral solutions containing chloride.

Transgranular stress corosion cracking
on a cold strip made of austenitic steel

Sectional view (50-fold enlargement)

Intergranular stress corosion cracking
on a cold strip made of austenitic steel

Sectional view (50-fold enlargement)

At temperatures above 100°C SCC can already be caused by very small concentrations of chloride or lye - the latter always leads to the trans­granular form.

Stress corrosion cracking takes the same forms in non-ferrous metals as in austenitic materials.

Damage caused by intergranular stress corrosion cracking can occur in nickel and nickel alloys in highly concentrated alkalis at temperatures above 400° C, and in solutions or water vapour containing hydrogen sulphide at temperatures above 250° C.

A careful choice of materials based on a detailed knowledge of the existing operating conditions is necessary to prevent from this type of corrosion damage.

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.

Because of the risk of crevice corrosion design and applications should be avoided which represent crevice or encourage deposits.

The resistance of high-alloy steels and Ni-based alloys to this type of corrosion increases in line with the molybdenum content of the materials.

Again the pitting index PRE (see Pitting corrosion) can be taken as a criteria for assessing the resistance to crevice corrosion.

Crevice corrosion on a cold strip
made from austenitic steel

Sectional view (50-fold enlargement)

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 this graph are mutually compatible; the anodic metal corrodes increasingly in line with the distance between two metals.

Galvanic potentials in seawater in relation to saturated calomel electrode
Source: DECHEMA-Werkstofftabellen (material tables)

Materials which can be encountered in both the active and passive state must also be taken into account. A CrNi alloy, for example, can be activated by mechanical damage to the surface, by deposits (diffusion of oxygen made more difficult) or by corrosion products on the surface of the material. This may result in a potential difference between the active and passive surfaces of the metal, and in material erosion (corrosion) if an electrolyte is present.

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 a spongy mass. The zinc either remains in solution or is separated 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.

Dezincing on a Copper-Zinc alloy
(Brass / CuZn37)

Sectional view (100-fold enlargement)

Conditions which encourage this type of corrosion include thick coatings from corrosion products, lime deposits from the water or other deposits of foreign bodies on the metal surface. Water with high chloride content at elevated temperature in conjunction with low flow velocities further the occurrence of dezincing.

Resistance table

The table below provides a summery of the resistance to different media for metal materials most commonly used for flexible elements.

The table has been drawn up on the basis of relevant sources in accordance with the state of the art; it makes jet no claims to completeness.

The main function of the table is to provide the user with an indication of which materials are suitable or of restricted suitability for the projected application, and which can be rejected right from the start.

The data con­stitutes recommendations only, for which no liability can be accepted.

The exact composition of the working medium, varying operating states and other boundary operating conditions must be taken into consideration when choosing the material.

Table keys
Assessment Corrosion behaviour Suitability
0 resistant suitable
1 uniform corrosion with reduction in thickness of up to
1 mm/year
     P risk of pitting corrosion
     S risk of stress corrosion cracking

restricted suitability

2 hardly resistant
uniform corrosion with reduction in thickness of more
than 1 mm/year up to 10 mm/year
not recommended
3 not resistant (different forms of corrosion) unsuitable

Table abbreviations:
dr: dry condition cs: cold-saturated (at room temperature)
mo: moist condition sa: saturated (at boiling point)
hy: hydrous solution bp: boiling point
me: melted adp: acid dew point

Resistance table

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