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Zinc Plating Processes
Zinc plating represents a valid alternative to galvanic treatments such as chrome plating or nickel plating.
Zinc is a blue-white metal, known since ancient times both as a pure element and as a component of Cu-Zn alloys (brass).
It is naturally abundant and it is mainly produced from the blende (ZnS) extraction.
After iron, zinc is the metal with the widest and most diversified use.
The usage as an efficient, cost- effective, lasting protective coating for ferrous materials is one of the main applications.
Zinc has an excellent protective power towards iron: for this particular property only the cadmium may compete with zinc, but the Cd toxicity and the current regulations limit its use only to the protection of articles that require high performance in very strict conditions (military sector).
There are therefore two protection mechanisms for galvanizing on iron:
1) A physical one due to the compact and thick zinc layer barrier effect, which isolates the underlying material from the corrosive environment;
2) an electrochemist one, which acts in areas where physical protection is lacking because of whatever reason.
In order to obtain good adhesion of the deposit to the substrate, as per all the galvanic treatments, the surface preparation is a very important aspect to be done.
Electrolytic coating with thicknesses of 5 - 12 Micron (with a maximum value of 30 Micron) is usually very uniform.
The thickness uniformity is a very important advantage, especially for sensitive and precise workpieces, whose tolerances may be maintained without the use of additional treatments.
The above mentioned feature makes galvanizing plating suitable for small and medium-sized pieces such as screws, pins, bolts ...
Electrolytic zinc plating is also applied to cover steel sheets and wires in the industrial sector. These treatments are carried out on plants working on a continuous cycle and with high producttivity. The results is the achievement, in a short time, of thick, uniform and corrosion-resistant deposits with a pleasant aesthetic appearance.
Since one of the main advantages of this treatment lies in the cost-effectiveness of the process, mechanical industry started prefering this treatment.
The meaning of galvanizing is the application of a zinc coating by electrodeposition in steady current regime, starting from a zinc salts aqueous solution.
Depending on the geometry and the dimensions of the pieces to be treated, static or barrel plants are used.
- Static zinc plating: the pieces to be galvanized are hooked on special plasticized structures -called "frames", which are the means by which the current is brought to the hung pieces on the units called bars.
This solution guarantees best quality in terms of corrosion resistance and aesthetic results.
It could be used for pieces of big dimensions or for delicate pieces with fine threads too.
- Barrel zinc plating: also by using automatic systems, the pieces are loaded into a plastic equipment (barrel), wich rotates around it own axis . The zinc is deposited by the passage of the elictricity through the barrel holes.
In order to avoid tangles and skeins during the rolling, the pieces contained in the barrel must have adequate geometry and weight.
The rolling itself can cause small dents on the pieces, whith the result of less corrosion resistance.
The barrel, unlike the frame, does not require manpower to load each piece individually.
Depending on the working temperature, galvanizing could be hot or cold.
The constantly growing demand of products protected from the corrosion has led, over the years, to the development of various galvanizing processes.
Please, find here below the used galvanizing baths:
Cyanoalkaline galvanizing baths:
Cyanoalkaline galvanizing was the first process used by industry for the zinc electrolytic deposition.
Despite the easy management, the great operational flexibility and the fact that it does not require a very accurate surface preparation, the new eco-environmental regulations and the greater sensitivity of operators to environmental safety issues have caused this process to be almost abandoned.
It is used only in case of low zinc and cyanide concentration, especially in big manual systems for the treatment of very bulky parts.
Cyanide-free alkaline baths:
They became into the market rather slow, bacause of problems of gloss and of the deposits passivation.
Today, after the overcoming of these limitations, the process is experiencing extraordinary success, especially in static zinc plating.
The solutions have a low metal content and are completely free from complexing agent, with interesting economic and operational advantages for the treatment and discharge of waste and washing water.
Acid galvanizing processes:
Since their first market introduction, the acid galvanizing baths have had a positive acceptance due to their advantages: greater conductivity and electricity saving, greater deposition speed, use of non-toxic products and, for the chloride baths, an aesthetic appearance of the deposits which is very closed to the nickel-chromium ones.
The main acid galvanizing processes can be grouped into two categories: sulphate and chloride Zn platings
Sulphate baths:
These baths are almost exclusively used for the galvanization of strip and iron wire on plants working on a continuous cycle.
Deposits are generally opaque or semi-gloss.
No bright additives are used; for some applications grain refining additives are used.
Chloride baths:
In 1970, after extensive research and because of ecological needs (the necessity to eliminate the cyanide from the electrolytic galvanizing baths), the first acid-bright galvanizing process was introduced on the market. The obtained deposits are very shiny and easy to be chromed. A very careful preparation is required. Eventual traces of engine oils, grease and oxides on the parts to be treated must be completely eliminated before the electrodeposition.
Features and advantages of these processes can be summarized as follows:
- significant shining and leveling power of the deposits;
- excellent yields and high deposition speed, higher than those optained by using other electrolytes;
- less hydrogen embrittlement;
- possibility of deposition on cast iron and alloyed steels.
With regards to disadvantages, the following are highlighted:
- eventual corrosion of the unprotected plants metal parts;
- need for accurate pre-treatment lines;
- more strained deposits;
- thicknesses distribution lower than the one obtained with alkaline solutions
- discontinuation of the processing because of periodic treatments with hydrogen peroxide for iron purification.
To learn more about the topic, please refer to the “Manuale Trattamenti e Finiture” - 2003 – Tecniche Nuove edition.
OMG FILTER PUMPS
The galvanizing treatment is quite widespread among our clients. OMG filter pumps can be used both with acid and alkaline solutions; in fact they are designed with the use of materials resistant to wear, to the action of time and the corrosive effect caused by the components of the galvanizing baths.
When Fe quantity in acid Zn baths is high, it is advisable clogging the filter papers with diatomaceous earth and carbon, using the same OMG filter pump; this operation allows to extend over time the use of the filter papers and to retain oily suspensions in the bath.
We also point out that the correct ratio of bath dimension / effective capacity is as follows:
***Static zinc plating: 1:3
***Roto zinc plating: at least 1:4 (even better 1:5)
The compliance with this ratio allows:
• At first to cut down the amount of Fe in the bath considerably and therefore to mantain it constant ;
• to replace filter papers much less frequently
A noteworthy aspect also concerns the construction materials of the OMG filter pumps: resistant to wear, to the time action and to acid and alkaline solutions.
The customer we would like to introduce in the newsletter, selected the filter pump model TE25 (effective capacity: 25 000 l / h, capacity: 56 000 l / h), correctly sized for its production needs. During the period 2013-2020 the clients bought 6 machines TE25 model
