This paper mainly introduces the application scope, metal plating treatment method, conventional chromium layer, test method and so on of the standard ISO 6158-1984 “Chromium plating for metal coating engineering”.
Subject content and scope of application
This standard specifies the representation method of chromium electroplating for engineering, the information to be provided by the demander to the supplier during processing and ordering, the heat treatment process specification of the workpiece before and after plating, the quality requirements of the plating, and the test method, etc.

ISO 6158-1984 Chromium electroplating for metal coating Engineering

This standard applies to aluminum electroplating for engineering on ferrous or nonferrous metal substrates with or without bottom coating. Engineering chromium plating is often referred to as “thick chromium” or “hard chromium” layer.

This standard does not include thin chrome surfaces for protective decorative purposes.

See Appendix A for examples and descriptions of applications of chromium for engineering.

2 Reference standards
GB1238 Metal coating and chemical treatment representation method

GB4955 Metal cover thickness measurement anode dissolved Coulomb method

Thickness of non-magnetic overlay on magnetic metal substrate – magnetic method

GB5270 Metal Coatings (electrodeposited and chemically deposited) on Metal substrates – Test method for Adhesion strength GB6458 Metal Coatings – Neutral Salt spray Test (Nss test)

GB6462 Method for microscopic measurement of cross-sectional thickness of metal and oxide coatings

Review of GB6463 metal and other inorganic coating thickness measurement methods

ISO 6158-1984 Chromium electroplating for metal coating Engineering

3 Terminology
Main surface: refers to some surfaces before and after plating on the workpiece, the coating on the surface of the workpiece for the appearance and/or performance is the main role.

4. Method of coating representation
The representation of chrome plating layer and related treatment shall be in accordance with GB1238.

Information TO BE PROVIDED BY THE DEMANDER TO THE SUPPLIER
The standard number of this standard;

b. The composition, content or brand of the matrix metal and its metallographic structure;

Whether stress relief treatment is required before plating;

d. Whether it is necessary to introduce compressive stress, such as shot peening before electroplating;

A detailed description of the main surfaces, all surfaces to which no coating should be applied, and all parts to which the chromium layer is allowed to extend can be made available with drawings or suitably marked samples;

f. any specific requirements or restrictions on pre-plating, such as replacing acid leaching with sandblasting;

g. If substratum is required, specify the type of substratum, such as nickel or crack-free chromium, etc.;

h. The minimum thickness of the chromium layer to be deposited, if required, the total thickness of the electroplating layer should be stated, if deemed necessary, the maximum thickness of the plating should be stated, especially the repair chromium plating of worn or out-of-tolerance parts. The thickness of the above coating refers to the thickness of the coating on the surface after polishing;

i. type of chromium layer such as porous, crack-free, microcracked, double-layered or conventional (see Sections 8.1 and 8.2);

j. The final finishing state of the chrome layer, such as plating, grinding or 1l1 row grinding (see Section 8.1.2);

k. Permitted surface defects, including type, size, location of defects;

l. any heat treatment requirements of the workpiece after electroplating;

m. Other special requirements.

Note :1) It is difficult to provide such data for the repair of old parts with chromium, and sometimes it is difficult to ensure the quality of the chromium layer.

ISO 6158-1984 Chromium electroplating for metal coating Engineering

Treatment of substrate metal before electroplating
6.1 Inspection before disposal

Prior to treatment, the supplier shall inspect the main surface to confirm whether there are obvious surface defects such as pores, cracks, and disallowed overburden, or any other defects that would be detrimental to the final finishing. All defects shall be brought to the attention of the purchaser prior to any treatment.

6.2 Stress Relief

If the Demander stipulates that the parts should be subjected to heat treatment to relieve stress before electroplating, it shall be carried out according to the working conditions in Table 1. Different working conditions can also be used, such as appropriately raising the temperature and shortening the time, but it needs to be based on effective implementation experience. Heat treatment should be performed prior to any pre-plating preparation and purification treatment with aqueous solution.

If stress relief is carried out after shot peening or other cold working process, the temperature should not exceed 220 ° C. Surface quenched workpiece, stress elimination should be treated at 130~150℃ at least 5h, if the hardness of the matrix surface is allowed to reduce, it can be treated at a higher temperature for a shorter time.

Whether non-ferrous metals require heat treatment to relieve stress shall be agreed upon by the supply and demand parties.

Heat treatment of steel parts before cadmium plating

6.3 Shot Peening

The introduction of compressive stress on the surface of unplated and to be plated workpieces usually improves the load and fatigue performance of these workpieces and partially compensates for the loss of fatigue strength by inhibiting the propagation of cracks from the coating into the base metal.

Compressive stress can be introduced by means of shot peening, and different metal materials are selected for different shot peening strength.

ISO 6158-1984 Chromium electroplating for metal coating Engineering

6.3.1 Steel

If it is necessary to use shot peening to improve the fatigue strength of steel workpieces, the shot peening strength (if not otherwise specified) shall be measured according to the method specified in Appendix c at least the arc height shall be:

0.3mm, for steel with tensile strength less than 1100MPa;

0.4mm, for steel with tensile strength equal to or greater than 1100Mpa.

On thin workpieces, in order to prevent deformation, a lower spray strength should be selected, so it may not be more satisfactory in avoiding fatigue strength reduction.

If no other provisions are made, the sprayed surface should be completely sprayed, that is, the shot marks should overlap with each other.

6.3.2 Non-ferrous metals

The shot peening strength of non-ferrous metals shall be specified by the user.

Heat treatment of workpiece after electroplating
If required by the Demander, the post-plating heat treatment shall be carried out in accordance with the provisions of Article 7.1 or 7.2. The heat treatment shall be carried out as soon as possible, generally not exceeding 4h, and shall be scheduled before polishing or other machining operations.

Heat treatment to eliminate hydrogen embrittleness of steel plating parts

7.1.1 See Table 2 for the heat treatment of steel parts after electroplating. Attention should be paid to:

The heat treatment of 400~480℃ is only suitable for those workpieces with fatigue failure tendency during use.

ISO 6158-1984 Chromium electroplating for metal coating Engineering

7.12 The heat treatment of workpieces shall not exceed their tempering temperature.

7.1.3 Surface quenched workpieces should be heated at 130~150℃ for no less than 5h. If the hardness of the matrix is allowed to decrease, it can also be treated at a higher temperature.

7.14 For workpieces without shot peening, the treatment temperature can be appropriately increased and the treatment time can be shortened, but it needs to be based on effective practical experience (see Chapter 6 and note to Table 2).

Heat treatment of steel plating

8 Requirements for coating
8.1 Conventional chrome layer

8.1.1 Appearance

When VIEWED by the NAKED eye, the main surface should be bright or shiny without pitting, blistering, peeling or any other defect that would adversely affect the final finishing condition. Chroma is not allowed on the surface of the workpiece that is used directly after plating or polished after plating, except for the edges.

The plated parts shall have no visible cracks, and the coating thickness greater than 50µm shall not have cracks reaching the substrate.

When visible blistering or cracking occurs during heat treatment or polishing, if this process is carried out by electroplating personnel, the coating shall be rejected as a reason.

ISO 6158-1984 Chromium electroplating for metal coating Engineering

8.1.2 Surface roughness

Whether there is a roughness requirement on the surface of the plated parts shall be specified by the demander. When there is a requirement, the profiler can be used to measure and rate. The acceptance level shall be agreed upon by the supply and demand parties.

8.1.3 Thickness

On the main surface, the minimum thickness of the chrome layer and bottom layer after finishing shall meet the requirements of the Demander (see h in Chapter 5):

The thickness of the coating is determined by the use characteristics of the chrome-plated workpiece. The application of chrome plating layer in industry is very complex, and it is difficult to strictly regulate the thickness of the coating. Appendix A provides some examples of chrome coating applications and a range of coating thickness options.

8.1.4 Hardness

Unless otherwise specified, the microhardness value Hv of the coating should not be lower than 750.

8.1.5 Bond strength

The chromium layer shall not be separated from the substrate when the test is carried out according to the method specified in Article 10.2. If there is a bottom layer, there should be no separation between the bottom layer and the matrix, or between the bottom layer and the chromium layer.

8.1.6 Number of pores per unit area (porosity)

Whether there is a requirement for porosity of the holes on the conventional chromium layer reaching the matrix shall be specified by the purchaser. See Article 10.3 for the test method. The level of assessment and acceptance of test results shall be agreed upon by the supply and demand parties.

8.2 Other types of chromium layers

ISO 6158-1984 Chromium electroplating for metal coating Engineering

8.2.1 General Provisions

According to the needs of different working conditions of the workpiece, in addition to the conventional chromium layer, other types of chromium coating can also be selected. However, these types of chromium layers may differ greatly from conventional chromium in appearance, hardness and other properties. Whether their quality is also required by the relevant provisions of Article 8l shall be agreed upon by the supply and demand parties.

If the demander requires the number of pores per unit area or cracks per unit length of other types of chromium layer, see Article 10.3 for the test method. The level of assessment and acceptance of test results shall be agreed upon by the supply and demand parties except for the relevant provisions of Article 8.2.2.

If the Demander needs to test the corrosion resistance of chromium without crack, double layer chromium or micro-crack chromium with nickel as the bottom layer, see Article 10.4 for the test method. The level of assessment and acceptance of test results shall be agreed upon by the supply and demand parties.

82.2 Characteristics and corresponding quality requirements of other types of chromium layers

8.2.2.1 “Crack free” chromium

Compared with conventional chromium, its hardness is low, brittleness is small, basically crack free, corrosion resistance is high, such as milk white chromium, the thickness of the chromium layer is generally not more than 25µm, and does not carry out finishing, also do not need to be on the high load surface, according to the provisions of Chapter 7 heat treatment, may have a negative impact on the corrosion resistance of this kind of coating.

ISO 6158-1984 Chromium electroplating for metal coating Engineering

8.2.2.2 Porous chromium

A mechanical, chemical or electrochemical treatment method can be used to obtain a porous chromium surface for oil storage, such as loose hole chromium. The demander should deal with the type of hole (such as point or groove), the density of the hole and related indicators

And the corresponding test method to make detailed provisions.

8.2.2.3 Cracked chromium

Different from the situation where cracks are formed in the conventional chromium layer, the crack network of cracked chromium is deliberately formed by using a specific process. Among them, the crack network of microcracked chromium is invisible to the naked eye, and there are more than 250 cracks per 1cm length in any direction of the whole main surface, while the number of cracks per 1cm length of coarse cracked chromium is usually much smaller than 250.

The hardness of cracked chromium is close to that of conventional chromium. Because cracked chromium has a cracked structure, it is easier to store oil. If nickel is used as the substrate, micro-crack chromium can improve corrosion resistance compared with coarse crack chromium.

8.2.2.4 Double layer chromium

Usually, a layer of conventional chromium is plated on the bottom crack free chromium, so its hardness is similar to that of conventional chromium, and at the same time has high corrosion resistance.

ISO 6158-1984 Chromium electroplating for metal coating Engineering

9 Take samples
Sampling inspection shall be carried out in accordance with the requirements of coating quality stipulated in Chapter 8. The sampling method and acceptance level shall be agreed upon by the supply and demand parties.

10 Test methods
10.1 Determination of coating thickness

To measure the thickness of the chromium coating at any position on the main surface, the measurement method used shall ensure that the measurement error is less than 10%.

10.1.1 Engineering tool measurement method

A direct reading of the thickness can be obtained by measuring the dimensions of the workpiece at a selected reference point before and after plating. Commonly used gauges are micrometer, depth gauge and so on. For the increase (outer circle) or decrease (inner hole) of the diameter size of the cylindrical workpiece, the thickness of the chromium layer can be represented only when the thickness of the coating is evenly distributed.

The uniformity of the thickness of the chromium layer can be measured in an appropriate manner. For example, the electroplating surface can be rotated around the axis of the workpiece to measure by a dial meter or by other indirect methods.

10.1.2 Magnetic method

When no nickel coating is used as the bottom layer, the thickness of chromium coating shall be measured according to GB4956.

10.13 Microscopical measurement of cross-sectional thickness

This method belongs to destructive thickness measurement method. Measure the thickness of chrome coating and other bottom coating according to GB6262.

ISO 6158-1984 Chromium electroplating for metal coating Engineering

10.1.4 Coulomb method

This method belongs to destructive thickness measurement method. Measure the thickness of chromium coating and nickel substrate according to GB4955. When the thickness of the coating exceeds 50µm, its reliability will be reduced.

In addition to the above thickness measurement methods, can also be used to measure the thickness of the profile size measurement method and other methods specified in GB6463 applicable to chromium coating.

10.2 Determination of bonding strength of coating

The bonding strength of the coating shall be checked according to the bending, grinding saw, thermal shock and other test methods specified in GB5270, and other applicable methods in this standard may also be used. For bending tests, the thickness of the chromium layer on the test piece should be 25µm.

It should be noted that various test methods for the bonding strength between the chromium coating and the matrix often have certain limitations, so the test method should be agreed by the supply and demand parties.

10.3 Determination of the number of pores or cracks in the coating

The detection of pores to the substrate in the conventional and “crack-free” chromium layers, as well as pores or cracks in other types of chromium layers, shall be carried out in accordance with the advertised method specified in Appendix D.

10.4 Test method for corrosion resistance of coating

Coating corrosion resistance test method, according to the provisions of GB6458.