Topics

Cleaning/Passivating rust-resistant steels:

Stainless steels (SS) with a chrome content of approx. 12% are able to spontaneously build up passive layer under humidity conditions. Stable passive alloys can be recognized by the fact that they have only a very low corrosion current density thereby ensuring that there can’t be any notable corrosion. Under these conditions the material forms a resistant, protective, but very thin chromium oxide layer on the surface. This barrier ensures that the external current conduction to the surrounding medium practically grinds to a halt. The material behaves in a similar manner as would a precious metal.

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Rouging:

The phenomena of rouging, which is named after the  rouge used by women, mostly occurs in hot ultra-pure water systems and clean steam systems. It is assumed that waters which are extremely lacking in ions have a “sucking effect” upon the ions in the alloy because they want to improve their ion poverty state. Furthermore the CO2-content can change the acidity of the water in these systems due to the corresponding carbonic acid equilibrium.

At the end there are system parameters, which constitutes the aggressiveness of these pure waters. Even ozone treatments promote the formation of thin rouging layers. However even in storage containers for CIP solutions (NaOH or KOH solutions) discolouring in the passive layer has been observed. If, for example, the rouging in pure steam systems is not removed from time to time, then blacking could occur. With increasing time pollution with small particles out of these layers can not be excluded.

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Blacking:

Blacking refers to the much thicker layers which can be formed in high purity steam systems the longer they are in operation. These matt black layers are generally firmly adhering layers of magnetite: Fe3O4.

At the beginning these are slightly transparent and can grow to a chalky surface structure. The high temperature of steam and the iron oxidation due to defects of chromium oxide layers influence respectively the formation of blacking. Different  impurities are also a source for the formation of layers, see chapter “cleaning /passivation”. In advanced stages the degradation of  surface layers may be observed and it could be possible that particles become detached.

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Lime scale/Corrosion in heating systems:

Various degrees of calciferous deposits can occur in raw water, which has not been separately treated, depending on the percentage of calcium salts (the degree of hardness) it contains and the temperature. These calciferous deposits, also known as boiler scale, usually occur above temperatures of 70°C and are related to escaping carbon dioxide and consequently exceeding the solubility level of the lime solution. Calciferous deposits are generally undesirable and can have the following effects:

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Types of corrosion/Corrosion removal

According to chemically free enthalpy as described in thermodynamics, it is the goal of every material to revert to its subordinate accidental state. That means it wants to revert to its component parts.

In the case of unalloyed steel this means that it will revert to iron oxide, from which it was originally formed with the help of a lot of energy. If unalloyed steel is left without protection in a humid environment for a longer period of time, then the iron will eventually be completely disintegrated.

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Material application/Choice of materials

The correct choice and application of suitable material are the fundamentals for using a component in the manner to which it is intended. When choosing it is very important to first define and understand the operating parameters as precisely as possible: e.g. the temperature, speed of flow, pH value and pressure.

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Treatment techniques/Cleaning processes

Besides choosing the ideal chemical treatment solution, the treatment process and the technology used has to ideally suit the system to be treated. To this end, over the past decades, Beratherm AG has continuously developed and refined specialized circulation and sluicing technologies.

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