Pretreatment

The accurate preparation of metal surfaces is crucial to ensure the reliability and durability of protective steel-zinc alloy, especially in the hot-dip galvanizing process. Upon entry into the galvanizing plant, raw metal faces show surface contaminants such as rust, scale, oils, paints, and residues from previous processes. These foreign elements, resulting from oxidation and other sources, can compromise the effectiveness and adhesion of the zinc coating, underscoring the need for meticulous preparation.

Contaminant removal is achieved through chemical treatments of degreasing, pickling, and fluxing. In cases where pieces are heavily marred by tenacious substances like paints, enamels, or anti-splash silicon resins used in welding, mechanical cleaning becomes equally essential. Although a preliminary visual examination may not immediately reveal all contaminants, precision in the preparation process is critical. Incomplete removal can lead to evident defects in the zinc coating, such as poor adhesion and dark burn marks. These flaws undermine corrosion resistance and aesthetic appearance of finished products. A synergy between chemical and mechanical treatments, coupled with rigorous quality control, both visual and instrumental, yields uniform and reliable results.

In summary, the meticulous preparation of metal prior to hot-dip galvanizing demands an integrated approach, wherein attention to detail and thorough cleaning take precedence. Safeguarding against contamination and implementing accurate preparatory processes are key to producing durable and high-quality finished products.

Degreasing

Degreasing baths remove oils and greases, coming from steel manufacturing, from metal surfaces. Alkaline baths, containing sodium hydroxide and reagents, emulsify oils and fats; temperature and time influence effectiveness. Acid baths, made from diluted acids, create stable emulsions needing less water.
Post-treatment washes prevent alkaline residues.
Heating is done using oil/gas or, more usually, from galvanizing furnace exhaust.
Efficacy of the degreasing bath lasts 1-2 years, sometimes over 5 years, prior it needs a complete regeneration.

Pickling

In the pretreatment process, the second step is the acid pickling, which removes deposits and rust from the pieces. Diluted hydrochloric acid is the most common, but sulfuric and hydrofluoric acids are also used. A galvanizing plant has pickling baths with varying concentrations. Inhibitors reduce the acid's aggressiveness and evaporation. Iron content increases over time, requiring replenishment with fresh acid. Heated enclosed tunnels reduce acid usage and allow higher concentrations. Emissions are treated by suitable abatement equipment.

Rinsing

After degreasing and pickling, steel is rinsed in temperature controlled water baths, preventing the transfer of acid and iron to the following stages. Post-pickling rinsing is essential for galvanizing, preserving subsequent treatments.

Fluxing

Fluxing is a crucial process in galvanization, essential for the subsequent dipping of the pieces in the molten zinc bath. It is necessary to remove oxides, facilitate adhesion, and provide corrosion protection. Ammonium chloride in the flux decomposes into ammonia and hydrochloric acid, contributing to pickling and removing occurring residual oxides. Fluxing salts reduce surface tension, facilitating adhesion between zinc and steel. The concentration and ratio of ammonium chloride to zinc chloride are crucial, influencing the effectiveness of the process and reducing smoke production.

ENVIRONMENTAL QUALITY

Pretreatment tunnel enclosure
with cleanbox™ technology

The enclosure of the pretreatment department inside a self-supporting structure, enables the confinment, extraction and disposal of acidic fumes and vapors generated during the immersion of materials in the tanks. The enclosure is designed under a zero-leakage principle isolating the pretreatment department from the rest of the galvanizing line, eliminating the risk of diffusion of fume throughout the plant and prioritizing the safety of workers and  equipment. This approach significantly reduces potential risks for operators and lowers maintenance expenses for equipment, leading to efficient operational spending and sustainability.

Highlights

Gas-leak-proof structure
The tunnel structure and the used materials have been redesigned, introducing the cleanbox™ technology that ensures an even better confinement solution against fumes and acid vapors providing quick and easy assembly of the tunnel at the same time.
The tunnel consists of a self-supporting light structure with removable ends built in sandwich-style polyurethane (PUR) panels with sides in fiberglass-reinforced PE equipped with automatic doors, and of a scrubbing tower.
This design allows both fast-building and extended lifetime of the structure.

Scrubbing unit
It dramatically reduces acid content in fumes and consists of a filtering tower equipped with inverter system for an automatic and smart regulation of energy consumption.

Anti-corrosion protection
Metal surfaces of the structure are finished with hard protection against acid corrosion.

Inspection windows
Allow the operators to overview the operations inside the tunnel, without entering it.

Rubber foam insulation
Crane runways and lifting equipment are equipped with rubber foam profiles to ensure complete insulation.

Tanks
High-grade polymer pretreatment tanks in standard and plus-size dimensions.

Chem distribution system
Two independent pumping and distributing systems for acid and alkaline chems eliminate the risk of mixing the solution of the degreasing tank with the solution of the pickling tanks.

Turbo-tank™
Available on request, turbo-tank™ allows to speed up the pickling process.

Automation and remote control
Pretreatment process may be completely managed by SCADA systems, ensuring continuous monitoring and improved productivity

Energy management
System optimizes exhaust capacity and blower speed, adjusting parameters as per operational needs to save power

Advantages

Top efficiency
Fumes cannot escape because the tunnel is always at negative pressure.

Energy saving
Only a scrubber needed, power is adjusted through inverter reducing consumption when all the doors are closed leading to energy saving.

Improved bearability
Tunnel provides comfort to workers by eliminating their exposure to acid vapors and fumes.

Protected environment
Equipment and environment in general are preserved from damages caused by acid fumes, promoting savings for equipment changeover.

Traceability
The confinement tunnel allows the use of automated hoists that ensure process accuracy and complete traceability of materials.

Quick return on investments

Quenching

A proper cooling process prevents excessive spreading of the zinc alloy and limits the dull gray appearance when high-silicon steels or high temperatures while galvanizing are used as well as prevents items from deforming ("banana effect").
Our galvanizing lines can be equipped with appropriate quenching tanks, efficiently and seamlessly integrated with dedicated material handling systems.

Passivation

Passivation is a post-galvanization treatment that significantly delays the formation of zinc hydroxide, commonly known as white rust.
This process is particularly employed to preserve the galvanized material, especially when subjected to extended storage periods and provides enhanced protection against atmospheric agents.