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How to check the quality of galvanizing

Release date:2018-06-28 author: click:

Galvanized with two types of hot dip galvanizing (hot dip galvanizing) and cold galvanizing (electrogalvanizing)

Hot-dip galvanizing has national standards, and cold-galvanizing is not recommended by the state because of its high pollution.

Our company is also in hot-dip galvanizing. In addition to thickness inspection, we also need to detect leakage plating, air bubbles, and surface bright plating (whether it is black and white). You can view the 13912 standard.

 

Metallic coatings Technical requirements for hot-dip galvanizing of steel products 13912-92

This standard refers to ISO

1459-1973(E) "Metal Overlays - Heat Resistant Zinc Anticorrosive Layers - Guiding Principles" and ISO

1461 - 1973 (E) "Metal Overlays - Hot Dip Galvanized Steel Products - Technical Conditions".

1 Subject content and scope of application

This standard specifies the technical requirements for hot-dip galvanizing on steel products.

This standard applies to the hot-dip galvanizing layer of steel products for corrosion protection.

This standard does not apply to hot-dip galvanized layers on unformed steel wire, pipe and sheet.

This standard does not stipulate the properties and surface state of the base material before hot-dip galvanizing. See Appendix A (Reference) for the condition of the base material that affects the hot-dip galvanizing effect.

This standard does not regulate the post-treatment of hot-dip galvanized products.

2 Reference standard

GB 2828 batch-by-batch inspection sampling procedure and sampling table (for continuous batch inspection)

GB 4956 Magnetic metal substrate thickness measurement of non-magnetic coatings Magnetic method

GB 12334 Metal and other inorganic coatings Definition and general rules for thickness measurement

 13825 metal coating quality determination of hot-dip galvanized layer weighing method

3 terms

3.1 Hot galvanizing

A process and method for forming a zinc-iron alloy or a zinc and zinc-iron alloy coating on a surface of a molten zinc liquid by immersing it in molten zinc.

3.2 Hot-dip galvanizing layer (abbreviation: plating)

A zinc-iron alloy or a zinc and zinc-iron alloy coating obtained by a hot-dip galvanizing method.

3.3 main surface

Refers to some surfaces on the part before hot-dip galvanizing and after hot-dip galvanizing. The coating on the surface plays a major role in the appearance and/or performance of the part.

3.4 Inspection lot (abbreviation: batch)

Hot-dip galvanized parts that are collected for sampling inspection.

In the hot dip galvanizing plant inspection, it refers to the same type and size of hot-dip galvanized parts produced in the same plating tank in a production class.

When the purchaser checks after delivery, it refers to one order or one delivery.

3.5 samples

A plated or plated set that is randomly selected from the batch.

3.6 Basic measurement surface

The area where the specified number of measurements are made on the main surface.

3.7 Local thickness

The arithmetic mean of the specified number of thickness measurements is made in the basic measurement plane.

3.8 average thickness

The arithmetic mean of the local thickness on a large piece or on all parts of the book.

4 Information required by the direction hot-dip galvanizing manufacturer

4.1 Required information

a. The standard number of this standard.

b. Composition and related properties of the base metal.

4.2 Additional Information

If necessary, the acquirer should provide the following information.

a. The main surface can be marked on the drawing or the sample with the appropriate mark;

b. Surface defects, which may be indicated on the drawings or may be indicated by other methods;

c. The appearance requirements of the coating may be described by samples or other methods;

d. Special requirements for plating thickness (see A3 in Appendix A);

e. Whether centrifugation is required and if required, can it be accepted for thickness (see Table 3).

5 Zinc content in zinc solution

The zinc content (mass) of the operating area in the galvanizing bath should not be lower than 98.5%.

6 sampling

When measuring the thickness of the coating, the number of parts of the sample shall be as specified in Table 1.

Table 1 Sampling requirements for thickness measurement

The number of parts in the batch The minimum number of parts in the sample

1∽3 all

4∽500 3

501∽1200 5

1201∽3200 8

3201∽10000 13

> 10000 20

6.1 If the main surface of the part is less than 0.001 m2, then Table 1 specifies the minimum number of basic measuring faces in the sample (see 7.3.3).

6.2 If the sample does not meet the requirements of 7.3, the number of parts of the original sample is doubled and then measured. If this larger sample can satisfy 7.3

For the requirements of the article, the batch of products is considered to meet the requirements; otherwise, the batch of products is a non-conforming product.

6.3 If the supplier and the buyer agree, the arbitration test can also be selected from GB 2828.

7 Requirements for plating

7.1 Appearance

All plated surfaces should be clean and undamaged. The main surface should be smooth, free of nodules, zinc ash and iron.

  

 2 Metallic coatings Technical requirements for hot-dip galvanizing of steel products

 A very small amount of storage and storage spots on the surface 1) should not be used as a reason for rejection.

Note: 1) Refers to the hot-dip galvanized parts during the storage and transportation process, due to the humid air in the environment, the formation of very shallow white spots on the surface of the plated parts.

Samples that indicate the appearance requirements of the coating shall be provided (or approved) by the purchaser if necessary.

7.2 Repair

Plates that fail to pass the visual inspection shall be repaired, but the total area of repair shall not exceed 0.5% of the main surface, and the single area shall not exceed

1dm2, otherwise it should be re-galvanized.

Different repair processes have different thickness requirements. When spraying zinc, the thickness of the coating in the repair area should meet Table 2 or Table 3.

Thickness requirements. When zinc-rich coatings and/or low-melting zinc alloys are used, the coating thickness shall be at least 50% of the minimum thickness in Table 2 or Table 3.

7.3 Thickness

In order to measure the exact thickness of the coating, both the supplier and the buyer should determine the size, location and quantity of the basic measuring surface according to the shape and size of the part.

When measured magnetically, the basic measuring surface shall not be less than 0.001 m2 and shall be measured at least 5 times in each basic measuring surface.

When measuring by the weighing method (arbitration method), the basic measuring surface is the area removed by one measurement and should not be less than 0.001 m2. Density of the coating

7.2 g/cm3, the approximate thickness of the coating can be calculated from the plating quality per unit area.

The thickness of the hot-dip galvanized layer should meet the requirements of Table 2 or Table 3.

7.3.1 The main surface is larger than 2m, the thickness requirement of the workpiece

In the sample, the average thickness of all the basic measuring faces on each part shall meet the average thickness requirements of Table 2 or Table 3.

7.3.2 Thickness requirements of the main surface at 0.001m2 to 2m2

Each part in the sample shall have at least one basic measuring surface. Each basic measuring surface shall meet the local thickness requirements of Table 2 or Table 3. The average of all local thicknesses in the sample shall satisfy the table.

2 or Table 3 average thickness requirements.

7.3.3 Thickness requirements of the main surface less than 0.001m2

Select a sufficient number of parts to form a basic measuring surface so that the basic measuring surface is not less than

0.001m2. Table 1 determines the number of basic measurement faces in the sample based on the size of the batch. The total number of parts measured is the product of the number of parts required for a basic measuring surface and the number of basic measuring faces. Each basic measurement surface should meet the table

2 or Table 3 local thickness requirements, the average of all local thicknesses in the sample should meet the average thickness requirements of Table 2 or Table 3.

If the wall thickness of the part is different, each thickness of the part shall be treated as a separate part when measuring the thickness of the part (see Table 2 or Table).

3).

Table 2 Thickness of hot-dip galvanized layer (when not centrifuged)

Parts and thickness mm Local thickness (minimum) Average thickness (minimum)

Steel parts > 6 70 85

> 3∽6 55 70

1.5∽3 45 55

< 1.5 35 45

Casting > 6 70 80

≤ 6 60 70

Table 3 Thickness of hot-dip galvanized layer (during centrifugation)

Part Size mm Local Thickness (Minimum) Average Thickness (Minimum)

Threaded parts ≥20 diameter > 10~< 20 diameter ≤10 453520 554525

Other parts (including castings) Thickness > 3 thickness ≤ 3 4535 5545

Note: The coating thickness requirements also apply to the gaskets associated with this.

7.4 Adhesion strength

The hot-dip galvanized layer should have sufficient adhesion strength, and the coating should not peel off when there is no external stress to bend or deform the plated part.

This standard does not stipulate the test method for adhesion strength.

When necessary, both the supplier and the buyer can agree on the requirements for the adhesion strength of the coating and the test method.

Appendix A Features of Parts Affecting Hot Dip Galvanizing

(reference piece)

A1 base metal

A1.1 material

Ordinary carbon steel and cast iron are suitable for hot-dip galvanizing, while sulfur-containing free-cutting steel is not suitable for hot-dip galvanizing.

A1.2 Surface condition

Before hot-dip galvanizing, in order to obtain a clean surface, the steel can be pickled after removing surface dirt and impurities such as grease, coating, welding slag, etc., and the casting can be treated by sand blasting (pill) or electrolytic etching.

A1.3 internal stress

During the hot dip galvanizing process, the deformation of the plated part may be caused by the elimination of stress in the base metal.

In order to avoid embrittlement of steel, steels corresponding to sensitive properties should be avoided as much as possible.

Heat treatment before hot-dip galvanizing can effectively avoid embrittlement of steel.

When the hardness of the steel is lower than 34 HRC, 340 HV or 325 HB, it is usually not brittle due to hydrogen permeation during pickling.

A2 design

The part design should be suitable for the hot dip galvanizing process.

A2.1 Tolerance When machining threads, the plating tolerances should be considered in order to conform to the assembly. In the bolted connection, the coating on the external thread has an electrochemical protection effect on the internal thread, and no galvanized layer is required on the internal thread. For internal threads, either tapping first or tapping after hot-dip galvanizing.

The thickness of the coating of the screw is related to the centrifugation. Centrifugation is done to obtain a smooth thread that meets tolerance requirements.

A2.2 closed cavity

For safety and ease of operation, the venting holes must be removed from the enclosed cavity. The closed cavity can cause an explosion during the hot dip galvanizing process. A3 Corrosion resistance and coating thickness The corrosion resistance of the coating is approximately proportional to the thickness of the coating. When used in a strong corrosive environment or requires a long service life, the technical requirements of the coating are negotiated by both the supplier and the buyer. A4 galvanizing bath

Usually, the zinc content (mass) of the operating area in the plating tank should not be lower than 98.5%. If there are special requirements, it should be prescribed by the purchaser. A5 post processing

The plated parts can be cooled in air or water after being taken out of the plating tank.

For small parts, the hot zinc can be immediately centrifuged to remove excess zinc.

A suitable surface treatment (for example) can block the storage and transport spots that may form on the surface of the plated part. Additional information: 

This standard was proposed by the People's Republic of China Machinery and Electronics.

This standard is under the jurisdiction of the National Technical Committee for Standardization of Metal and Non-Metallic Coatings.

This standard was drafted by the Wuhan Institute of Mechanical Materials Expansion and Expansion.

The main drafters of this standard are Huang Yerong, Li Zhi and Ni Haoming.