Electrolytic galvanizing process without hydrogen formation.

The present patent application for industrial invention relates to an electrolytic galvanizing process without hydrogen formation.

As it is known, galvanizing is the process used to apply a zinc coating on a metal part, generally a steel part, to protect it against galvanic corrosion. The galvanizing process can be made according to different methodologies, such as:

• hot galvanizing;

• electrolytic galvanizing;

Hot galvanizing generally refers to immersion in melted zinc at an average temperature of 455° C. In this phase, in addition to covering steel, zinc forms an alloy with the surface layer, giving mechanical resistance and grip to the treated material.

The process can be divided into the following separate phases:

• pickling and degreasing with chloridic acid and surface-active agents at ambient temperature;

• flushing: immersion in ammonium chloride and zinc chloride solution;

• galvanizing: after pre-heating a 100 ° C, immersion in tub with melted zinc at 455 °C for time needed by steel to reach the same temperature as zinc.

In case of electrolytic galvanizing, as mentioned above, the material to be treated is suitably prepared (pre-degreased, pickled and degreased). From now onwards the process is different, since the material is immediately immersed in an electrolytic solution containing zinc salts. Finally, a current passage is created between part and solution, causing the metallic zinc to deposit on the part surface. The galvanizing process of the invention is of electrolytic type and is specifically designed to avoid the spontaneous formation of hydrogen during the galvanizing process, being it a frequent, rather dangerous phenomenon that causes the weakening of galvanized parts.

In order to solve this serious problem, it is now recommended that the parts coming out of the galvanizing process immediately undergo a thermal treatment known as "baking".

During the baking process, within four hours and preferably within one hour from the electrolytic coating treatment, the galvanized material is brought to a temperature of about two hundred degrees Centigrade for a minimum of two hours.

The ISO 9588 standard recommends to carry out said baking process before the chromatizing treatment (or passivation) in order to avoid losing its efficacy, since all passivation treatments, which are extremely important in terms of salt spray resistance (appearance of white spots), cannot withstand a thermal shock higher than 190 °C - 220 °C.

Moreover, the standard recommends that the duration of the baking process must be at least four hours in order to harmonize the temperature of treated parts as much as possible.

In any case, said baking process can attenuate said hydrogen-caused weakening effect, but cannot eliminate it completely, as confirmed by the standard UNI EN ISO 4042:2003.

EP0545089 discloses an additive composition used in galvanic acid baths containing zinc or a mixture of zinc ions and at least one additional metal chosen between nickel and cobalt. The galvanic baths also contain chloride ions or a mixture of chloride ions and sulphate.

EP0498436 discloses a zinc electroplating process of an aluminum strip. This process can be made continuously at high speed and high current density. According to this process, the aluminum strip undergoes anodic electrolysis before cathodic electrolysis in zinc acid.

As mentioned above, the main purpose of the present invention is to devise a new electrolytic galvanizing process in which all treatment phases are free from the risk of causing spontaneous hydrogen formation, thus eliminating the need for successive baking treatments.

This purpose has been completely achieved with the new electrolytic galvanizing process of the invention, the innovative characteristics of which are described in the attached claims.

In order to achieve the aforesaid purpose, some changes were introduced compared to the traditional electrolytic galvanizing process, both with reference to the preparation phase used to clean the parts to be galvanized and the final galvanizing phase.

It must noted that the preparation phase of a traditional electrolytic galvanizing process includes a first chemical anodic degreasing treatment, followed by a second electrolytic degreasing treatment with an alkaline liquid submitted to a voltage of 10/12 volts for approximately two minutes.

According to the galvanizing process of the invention, the two successive degreasing treatments were eliminated and replaced by other treatments because it was observed that they favored the undesired formation of hydrogen that tends to infiltrate and stagnate in the internal and surface interstices of the parts, thus causing the aforesaid weakening.

During the galvanizing process of the invention the material is degreased with a mechanical-chemical treatment. The rotobarrel loaded with parts to be galvanized is immersed for a predefined time into a tub that contains chemical degreasing agents that are able to clean the parts without minimally impairing their mechanical resistance features.

Said mechanical action originates from the continuous chaotic mixing of parts inside the rotobarrel. Moreover, the galvanizing process of the invention is different for the implementation of the pickling phase amongst the preparation treatments before the final galvanizing phase.

The pickling tub used in traditional galvanizing processes has a high (higher than 30%) concentration of acid (specifically hydrochloric and sulphuric acid) at a temperature of about 35-45° C, whereas the acid concentration in the pickling tub used in the galvanizing process of the invention is reduced to approximately 10% at ambient temperature.

Instead, special parameter values have been chosen for the final galvanizing phase, such as bath time, temperature and pH, chloride and acid concentration in said bath, and working voltage of rectifiers.

More precisely, the following values have been chosen:

- working voltage of rectifiers not higher than 6 volts;

- permanence time in tub from 45 to 120 minutes;

- maximum temperature of galvanic bath between 35 and 40 ° C;

- bath pH between 5.5 and 6;

- potassium chloride concentration in tub between 150/180 g/l;

- zinc chloride concentration between 35/50 g/l;

- boric acid concentration between 20/30 g/l;

In brief, it can be said that the galvanizing process of the invention comprises a preparation phase of the parts and a final galvanizing phase, said preparation phase being characterized in that it provides for:

- a mechanical-chemical degreasing treatment, during which the rotobarrel loaded with parts is immersed in a tub containing chemical degreasing substances, of known type, wherein other chemical substances, such as surface-active agents, are present in addition to 30-40% caustic soda (of the volume of liquid contained in the tub) in order to eliminate various types of oils. - a pickling treatment during which the rotobarrel loaded with parts is immersed in a tub containing hydrochloric acid with approximately 10% concentration at ambient temperature for 7/12 minutes.

On the other hand, the final galvanizing phase is characterized in that:

- working voltage of rectifiers is not higher than 6 volts;

- the permanence time in tub varies from 45 to 120 minutes;

- the maximum temperature of galvanic bath is 35/40 °C;

- the bath pH is comprised from 5.5 and 6;

- potassium chloride concentration in tub is 50/180 g/l;

- zinc chloride concentration is 35/50 g/l;

- boric acid concentration is 20/30 g/l;