First of all, hot dip galvanizing is mainly used in the anti-corrosion treatment of steel materials, right? Then its specification should include international standards, national standards, such as ISO, ASTM, GB and so on. There are also process processes, such as pre-treatment, galvanized, post-treatment steps.

Pretreatment includes degreasing, pickling, plating aid and drying. Degreasing is to remove surface oil, pickling is to remove oxidation and rust, plating may need to use zinc ammonium chloride solution, drying temperature and time need to be controlled. In the galvanizing process, the temperature of the zinc liquid is generally 445-465 ° C, and the dipping time is determined according to the thickness of the workpiece, which may take 1-10 minutes. Post-treatment includes cooling, passivation and trimming. The passivation treatment can be chromate or chromium-free passivation, and the cooling method is water cooling or air cooling. In terms of quality control, coating thickness, adhesion, appearance need to be tested, and possibly salt spray test.

Safety and environmental protection should pay attention to ventilation, waste water treatment, personal protection. Applications include power, transportation, construction and other fields. Common problems may include missing plating, uneven coating, zinc nodules, etc. The solution may involve pre-adjustment treatment or zinc solution composition.

However, there may be some missing details. For example, what is the difference between different international standards, such as ISO 1461 and ASTM A123/A123M? In addition, is there a different type of plating aid in the pre-treatment? What are the advantages and disadvantages of chromium-free and chromate passivation in passivation treatment? How to measure coating thickness in quality control, such as magnetic thickness gauge or metallographic microscope? In terms of environmental protection, what are the specific standards for wastewater treatment?

It may also be necessary to consider whether galvanized workpieces require additional tests, such as bending tests or impact tests, to detect coating adhesion. In addition, is the thickness of the galvanized layer related to the environment in which the workpiece is used, such as whether a thicker coating is needed in the Marine environment?

There are also some practical application problems, such as whether there is a difference between the galvanizing process of large structural parts and small parts? How to deal with the formation of zinc slag in the process of galvanizing? What is the effect of aluminum content in zinc solution on coating?

It may also be necessary to add some common domestic standards, such as the specific content of GB/T 13912, and what are the characteristics compared with the standards of other countries. In addition, what are the advantages and disadvantages of hot-dip galvanizing compared with other anti-corrosion methods (such as electrogalvanizing, spraying)?

In terms of safety, besides ventilation and personal protection, are there any other considerations that need attention, such as fire risk? Because zinc may release harmful gases at high temperatures, how to deal with emergencies?

To sum up, I need to know in more detail the specific requirements of the individual standards, the selection of chemical agents for the different pre-treatment steps, the precise control of parameters during the galvanizing process, the different methods of post-treatment and their effects, the specific methods and standards for quality testing, and the specific measures in terms of environmental protection and safety. It may also be necessary to add some practical examples or solutions to common problems, such as how to deal with brittleness caused by excessive coating, or how to repair areas that are missing plating.

Hot-dip galvanizing is a widely used steel anticorrosion process, its specification involves many aspects, including international standards, process flow, quality control and environmental protection and safety.

1. International and national standards

ISO 1461: Specifies the minimum thickness of the galvanized layer (depending on the thickness of the workpiece), such as the workpiece thickness ≤6mm, the coating thickness ≥70μm.

ASTM A123/A123M: Applicable to structural steels where the coating thickness is required to be related to the thickness of the steel, e.g. 85μm if the steel thickness is greater than 6mm.

GB/T 13912: Chinese national standard, similar to ISO 1461, emphasizing coating uniformity and adhesion.

EN ISO 1461: European standard, consistent with ISO, which adds requirements for the appearance of plated parts.

2. Detailed explanation of process flow

Pre-treatment:

Degreasing: Alkaline solution (such as NaOH) to remove grease, temperature 60-80℃, time 10-30 minutes.

Pickling: hydrochloric acid (concentration 10-20%) or sulfuric acid, remove the oxide, need to control the acid concentration and temperature to prevent overcorrosion.

Plating aid: zinc ammonium chloride solution (pH 3-4), to prevent oxidation, drying temperature of about 80-120℃.

Hot dip galvanizing:

Zinc liquid composition: pure zinc (≥98%), adding aluminum (≤0.02%) can improve the gloss of the coating, lead (≤1%) reduce zinc slag.

Temperature control: 445-465℃, too high leads to an increase in zinc slag, too low affects fluidity.

Dipping time: 1-10 minutes, depending on workpiece thickness and structural complexity.

Post-processing:

Cooling: air cooling or water cooling (water cooling can refine the coating structure).

Passivation: chromate passivation (good corrosion resistance, but poor environmental protection) or chromium-free passivation (environmental protection, such as silicate or molybdate).

Trimming: remove zinc nodules and burrs, and supplement zinc if necessary.

3. Quality control and testing

Thickness measurement: A magnetic thickness gauge or metallographic microscope, ISO 1461 requires at least three measurements to be averaged.

Adhesion test: grid test or hammer test, plating without stripping is qualified.

Appearance check: no leakage plating, foaming, allowing slight color difference and zinc pattern.

Salt spray test: According to ASTM B117, the corrosion resistance time of the galvanized layer is usually ≥100 hours.

4. Environmental protection and safety

Waste gas treatment: The HCl gas produced by pickling needs to be neutralized by lye; Exhaust system is arranged around the zinc pan.

Wastewater treatment: Wastewater containing acid and heavy metals needs to be neutralized and precipitated, in line with GB 8978 discharge standards.

Personal protection: acid and alkali resistant gloves, goggles, gas mask; High temperature operation to prevent burns.

5. Common problems and solutions

Leakage plating: Check whether the pretreatment is thorough and whether the plating aid is ineffective.

Zinc nodules: Adjust plating speed or zinc solution composition (e.g. reduce lead content).

Coating brittleness: control the temperature of zinc liquid to avoid too thick coating.

6. Application and comparison

Application: power tower, highway guardrail, building steel structure (especially the Marine environment needs to be thickened).

Comparative advantages: more corrosion resistance than galvanizing, life span of 20-50 years; Stronger adhesion than spraying.

7. Special consideration

Large workpieces: need to be dipped in sections, control the lifting Angle to reduce zinc residue.

Zinc slag treatment: regular dredging slag, can be recycled.

Through the above specifications, hot dip galvanizing has become the preferred process for steel protection while ensuring anti-corrosion performance, taking into account production efficiency and environmental protection requirements. In practical applications, parameters should be adjusted according to specific standards, and the process should be continuously optimized to meet different environmental requirements.