Home appliance industry requires food-grade materials with good resistance to corrosion phenomena, the most common coating used for this application is the enamel. The porcelain (vitreous) coating is applied to the steel by a high productivity industrial enamelling process that can be affected by several defects among them the fishscaling, that is considered the most dangerous damage in the production of enamelled steel products. Fishscaling is caused by an excess of hydrogen content in the steel that is produced during enamel firing at a temperature of 800-850°C when hydrogen dissolves into the steel. The solubility of the hydrogen in steel strongly decreases during subsequent cooling but the hydrogen present tends to slowly migrate towards the enamel-steel interface. In the interface region hydrogen concentrates as the ceramic layer prevents the diffusion, thus producing high pressure. As a consequence small chips of enamel pop loose from the layer even after a long time after the end of the product manufacturing. Steel susceptibility to fishscale depends on its microstructure in particular on the content, distribution and nature of site traps that are available for trapping the hydrogen thus preventing its diffusion toward the surface. It was demonstrated that steel resistance to fishscale strongly depends on specific parameters both of manufacturing process and chemical composition of the steel. Steelmakers and Home Appliance industry have to perform quality control of the sheet steel to prevent fishscale defects.


HELIOS 2 FISHSCALE is an innovative laboratory instrument that can measure hydrogen diffusion coefficient through any metallic sheet. HELIOS 2 FISHSCALE can be used as:

-  laboratory instrument to evaluate the fish-scale susceptibility in steel for enamelling;

-  laboratory instrument to compare enamel performance during forced fish-scale test.

The test method is based on the elaboration of the permeation curve obtained for the examined sheet metal, according to ASTM G148-97(2011) Standard. The most common technique for fishscaling assessment is that defined by the standard EN10209. The standard uses the “TH” parameter (a function of the break through time, t0) to determine the fishscale susceptibility of steels. The break through time is the simplest method to get an indication of steel susceptibility to fishscaling, anyway the method is not valid for some steels (for instance for Titanium and Boron steels). Moreover the method  is based on a partial measurement of the permeation curve, so it does not provide specific indications on steel trapping ability. HELIOS II Fishscale overcomes these limits and it is able to perform test using a chemical substances not restricted by the current REACH norm (as arsenic, or mercuric chloride, which are indicated in the EN10209:2014 Standard).

Once acquired, the signal that depends on the hydrogen flux through the examined test sample, is processed by proper mathematical codes (on by means of a dedicated panel PC) and an index that the fish-scale defect appears is evaluated are evaluated. HELIOS II 2-PERMEATION is a completely automated instrument!

Letomec offers customized instrument and consultancy to define optimal hydrogen charging conditions (current density, electrochemical solution, re-circulation of the test solution, air circulation, etc.).

HELIOS method is a part of the UNI 11734 Standard. Moreover, HELIOS method could be also part of the EN 10209 standard, which is currently under review.


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HELIOS MAGMA, is an innovative equipment under development. It is meant to achieve a real time monitoring of the furnace atmosphere in order to return an alarm in case of presence of anomalous amount of certain substances. The main aim is the prevention of dangerous situations and production scraps.

HELIOS MAGMA can be applied to:

-Hot Stamping furnace for Automotive Industry;

-Enamelling furnace for home appliance industry.