Metallic coatings | Electroplated coatings of chromium for engineering purposes
1DScope and field of application
5DInformation to be supplied to the electroplater
6DTreatment of basis metal before
8DHeat treatment after electroplating
Electroplated coatings of chromium used for engineering purposes are much thicker than those used for decorative purposes. They are often called "heavy Chromium" or ''hard chromium'' coatings.
Chromium coatings are used for engineering purposes in industry mainly to take advantage of one or more of the following properties:
(a)@low coefficient of friction;
ISO 1456 and ISO 1457 specify requirements for coatings intended primarily for decorative purposes.
It should be recognized that whilst electrodeposited chromium is hard, it is also brittle. This means that coatings will not permit significant deformation without cracking or spalling and therefore the following precautions should be observed :
(a)@the basis metal should be such that it will resist all applied stresses h depth;
(b)@under conditions of deformation or shock loading, the thickness of the coating should be kept to a minimum.
Close liaison between designers, manufacturers, electroplaters and purchasers is desirable in order to obtain satisfactory electroplating and to avoid any adverse effects on the mechanical properties of the article. The introduction of compressive stresses into the surface of components is generally beneficial for both sustained load and fatigue properties of un-plated components and on subsequently electroplated components, and partially offsets loss of fatigue strength by delaying crack propagation from the coating into the basis metal.
The thickness of chromium to be applied will depend upon the properties required and the particular application. 1n view of the wide variety of industrial uses of chromium coatings, it is not possible to specify thick nesses but the foIIowing is intended to indicate typical practice.
a) Coatings normally used h the as-plated condition or finished by polishing or honing only :
1)@Coatings up to 12 lm thick.
Examples of use:
(i)@plastics moulds, where chromium provides a surface that resists corrosion and allows easy release of the product;
(ii)@cutting toots, taps, dies, drills, etc., where the chromium coating prevents metal removed by machining from adhering behind the cutting edge.
2)@Coatings 12 to 50 Κm thick.
Examples of use:
(i)@rams for use in hydraulic equipment;
(ii)@cylinder liners for internal combustion engines.
3)@Coatings over 50 Κm thick.
Examples of use:
Prevention of corrosion and/or wear on articles where finish and close limits are not of importance.
b) Coatings finished to size by grinding
The hardness of chromium is such that the only practicable method of finishing to size is by grinding or by a similar procedure.
Examples of use:
(1)@on new articles : the thickness of chromium remaining after grinding should be specified by the purchaser. The recommended finished thickness is 50 to 250Κm .
(2)@for reclamation of worn or over-machined articles(see annex B) : the thickness required wilt be that needed to replace lost metal. lf heavy coatings are necessary. it is possible to deposit sufficient thickness of an undercoat metal, for example nickel, such that, after grinding, the final chromium coating will be 100 to250 pm thick. If using chromium alone, the maximum thickness to be applied will be limited by technical and economic requirements.
c) Use of undercoats
Nickel undercoats are recommended for protection against severe corrosion, and tests for the corrosion resistance of the nickel alone may be desirable. Nickel undercoats should be h accordance with ISO 4526. For special requirements, other metals may be used as undercoats.
This International Standard specifies requirements for electroplated coatings of chromium , With or without undercoats, on ferrous and non-ferrous metals for engineering purposes.
ISO 468, Surface roughness | Parameters, their values and general rules for specifying surfaces.
ISO 1456, Metallic coatings | Electroplated coatings of nickel plus chromium.
ISO 1457, Metallic coatings | Electroplated coatings of copper plus nickel plus chromium on iron or steel.
ISO 1463, Metallic and oxide coatings | Measurement of coating thickness - Microscopical method.
ISO 2177, Metallic coatings - Measurement of coating thickness | Coulometric method by anodic dissolution.1)
ISO 2178, Non-magnetic coatings on magnetic substrates | Measurement of coating thickness - Magnetic method.
ISO 2819, Metallic coatings on metallic substrates | Electrodeposited and chemically deposited coatings | Review of methods available for testing adhesion.
ISO 4516, Metallic and related coatings | Vickers and Knoop microhardness tests.
ISO 4518, Metallic coatings@- Measurement of coating thickness | Profilometric method.
ISO 4519, Electrodeposited metallic coatings and related finishes | Sampling procedures for inspection by attributes.
ISO 4526, Metallic coatings | Electroplated coatings of nickel for engineering Purposes.
For the purpose of this International Standard, the following definition applies.
Significant surface : The part of the article covered or to be covered by the coating and for which the coating is essential for serviceability and/or appearance.
lf required, select a random sample of the size required by ISO 4519 from the inspection lot. Inspect the articles in this sample for conformity with the requirements of this International Standard and classify the lot as conforming or not conforming to each requirement according to the criteria of the sampling plans in ISO 4519.
The electroplater shall be supplied with the following information:
The significant surface shall be examined by the electroplater for visible surface defects, such as porosity, cracks and undesirable coatings or any other defects detrimental to the final finish. Any defects shall be brought to the attention of the purchaser prior to any processing.
6.2 Stress relief
Before being electroplated, parts shall, if specified, be stress relieved. The conditions set out in table 1 shall normally be used, but different conditions, that is, suitable combinations of a shorter time at appropriate higher temperatures may be used if they have been shown to be effective. The heat treatment shall be carried out before the commencement of any preparation or cleaning treatment using aqueous solutions.
Table1- Stress relief conditions before electroplating
lf stress relief a performed after peening or other cold working processes, the temperature shall not exceed 220 QC. Parts with surface-hardened areas shall be stress relieved at 130 to l50 0Cfor not less than 5 h, but shorter times at higher temperatures may be used if the resulting loss of surface hardness of the substrate is acceptable.
Stress-relieving heat treatment is not normally required for non-ferrous metals.
lf peening is necessary to improve the fatigue strength, the peening intensity, unless otherwise specified, shall be such that, when measured by the method described in annex A, the arc height is at least
- 0.3mm for steers of tensile strength less than1 100 MPa;
- 0.4mm for steels of tensile strength 1 100MPa or greater.
NOTE - Lower intensities may be necessary on the sections to avoid distortion, but may not be MIY effective h avoiding toss h fatigue strength.
Unless otherwise specified, the peening shall be performed so that the area concerned is completely covered, i.e. the ball marks completely overlap each other.
6.3.2 Non-ferrous metals
For non-ferrous metals, the peening intensity shall be specified by the purchaser.
7.1 Appearance of regular chromium
The significant surface shall be bright or lustrous and, when inspected by the naked eye, Shall be free from pits, blisters and exfoliation or any Other defects detrimental to the final finish. Nodular growths elsewhere than at the extreme edges of coatings are not permitted on articles used in the as-plated condition or on the surface of ground articles.
Any electroplated articles shall be free from cracks visible to the naked eye. Coatings thicker than 50 pm shall be free from cracks extending to the basis metal.
Blisters or cracks produced by heat treatment or grinding procedures, if carried out by the electroplater, and that are visible to the naked eye, shall be cause for rejection.
7.1.2 Surface finish
lf a specified final surface roughness is required, the method of measurement shall be as specified h ISO 468.
NOTE - As a guide, for ground finishes 0.4Κm Ra may be regarded as a ''commercial finish" and 0,2 L[m Ra as a ''good commercial finish''.
The finished minimum thickness of chromium on the significant surface, and of any undercoat(s), shall be as specified by the purchaser [see clause 5, item g].
The hardness of the coating, unless otherwise specified, shall be not lower than 750 HV when measured by the method specified h ISO 4516.
NOTE - Some softening of the coating may occur during heat treatment.
No un7versaIIY Satisfactory test for th3 adhesion of chromium coatings to the basis metal is known. However, a bend test on a representative sample, electroplated with 25Κm of chromium, can be applied as a test of the effectiveness of the process. A review of methods of testing adhesion is given in ISO 2819, which includes a thermal shock test that has been found applicable h certain cases.
7.2 Othertypes of chromium
For special applications, coatings other than regular chromiummay be required.
Examples of these are as follows.
If required by the purchaser, heat treatment after electroplating shall be performed as described h 8.2 and 8.3. The heat treatment shall be performed as soon as possible and not later than4 h after electroplating and before any grinding or other mechanical finishing operation.
8.2 Heat treatment of steels for reduction of hydrogen embrittlement
The heat treatment of electroplated steel articles for the reduction of hydrogen embrittlement shall be in accordance with there requirements given h table 2, but heat treatment at 400 to480 0C is applicable only of Parts that are liable to fatigue in service.
Articles shall not be heat treated above their tempering temperature.
Unpeened parts may be heated for shorter periods at a higher temperature if the conditions have been shown to be effective(see clause 6 and the note to table 2).
If nickel undercoats are used, heat treatment shall be carried out in accordance with ISO 4526 after nickel electroplating. For nickel undercoats thhlner than 25 pm, a second heat treatment shall be carried out in accordance with table 2 after chromium electroplating.
Parts having surface-hardened areas shall be heated at 130 to150 0C for not less than 5 h or at a higher temperature if the resultant Toss of hardness of the substrate is acceptable.
8.3 Heat treatment of aluminium and aluminium alloys to improve adhesion
It should be noted that the mechanical properties of some aluminium alloys may be adversely affected by heating. If heat treatment can be carried out and is required to give the desired adhesion on alumhium or aluminium alloys, the electroplated articles shall be heated in air to give a temperature rise of 2 to3 K/min until a temperature of 130 to 140 OC is attained. This temperature shall then be maintained for a period of not lessthan2 h and not more than 3 h.
9.1 Determination of coating thickness
Measure the thickness at any Place on the significant surface designated by the purchaser. Use a method capable of giving a measurement uncertainty of less than 10 %.
9.1.1 Direct measurement
Provided that there is a reference pout, measurement before and after electroplating provides a direct reading of thickness. For this purpose, normal engineering instruments, for example micrometers, depth gauges etc., can be used. Measurement of the increase in diameter of cylindrical parts does not provide a true reading of thickness unless the distribution of the deposit is even. Such distribution can be checked, for example, by rotating between centers and using a dial indicator or by the use of indirect methods of measurement.
9.1.2 Magnetic method
Use the method specified in ISO 2178 for measuring the thickness on magnetic basis metals.
9.1.3 Metallographic sectioning and microscopical method
Use the method specified in ISO 1463.
9.1.4 Coulometric method
Use the method specified h ISO 2177.
This method becomes decreasingly reliable for thicknesses over50Κm . The test conditions and electrolyte are dependent on the test equipment used and the relevant information should be obtained from the manufacturer's instructions.
9.1.5 Profilometric method
Use the method specified in ISO 4518.
9.2 Visual examination for cracks (not applicable to regular chromium)
Examine the surface for cracks in reflected 1ight under an optical microscope at a magnification of X 100. For very fine crack patterns and accurate counting, a higher magnification may be desirable. Use a micrometer eyepiece or similar device for indicating the distance over which cracks are counted.
According to the magnification used, take a measured length such that at least 40 cracks are counted.
9.3 Copper deposition for determinnation of cracks or pores
Electrodeposition of copper from an acidic sulfate solution at low current density or low voltage,' deposition of copper occurs only On the underlying metal which is exposed through cracks, pores and other discontinuities.
This method provides a rapid means of visual inspection for uniformity Of cracks or pores but, if counting of cracks or pores is required, a microscope should be employed as describedin9.2.
Best results are obtained if the test is performed immediately on completion of electroplating. If there is any delay, the roughly degrease the specimen, avoiding the use of any electrolytic treatment. Ultrasonic cleaning or rubbing with an aqueous paste of levigated magnesia are examples of suitable methods.
Deposit copper for approximately 1 min at room temperature from a bath containing approximately 200 g of copper(ll) sulfate pentahydrate (CuSO4.5H20) and 20 g of sulfuric acid per liter, at an average current density Of 30 A/m2.
NOTE - The test specimen and the anode should be connected to the current supply before immersion.
In cases when the test is applied several days after chromium deposition, immerse the specimen h a solution containing 10 to20 g of nitr7c acid per liter for 4 min at approximately 95 0Cbefore deposition of copper, to help reveal the cracks or pores.
Table 2 - Treatment of steers after electroplating
Use a test specimen from carbon steel sheet1). of hardness range 400 HV 30 to 500 HV 30 and of thickness 1,6 mm, which has beencuttoasizeof75 mm x (20 jE 0,2) mm and ground to a thickness of 1,3 +- 0,02mm.
The deviation from flatness shall not exceed an arc he;9ht of 38 pm when measured as described below. With the specimen rigidlyheld in the fixture shown h the figure, peen it on the exposed side for the same period and under the same conditions as the part to be electroplated.
After peening, remove the specimen from the fixture and measure the curvature of the unpeened surface with a depth gauge. The specimen being supported on four 5 mm diameter balls, forming a rectangle 32 mm x 16 mm. Align the gauge symmetrically On the specimen with its centre stylus at the centre of the specimen. Measure the arc height at the centre of the specimen over the gauge length of 32 mm, measuring to the nearest 25 pm. Then ad1'USt the peening conditions if necessary, to give the required arc height.
Figure - fixture for peening test speciments
Before reclamation, cons7derat;on should be given to the need for crack detection of parts. Complete removal of the chromium from worn Chromium electroplated parts and from Parts electroplated with insufficient chromium is recommended before electroplating in accordance with this International Standard.
Nevertheless, with special pre-treatment, worn or undersized sound chromium coatings may be hunt up without stripping. This should be carried out after cleaning and anodically etching h the electroplating bath at normal electroplating current density for 10 to 20 s. The direction of the current flow shall then be immediately reversed and normal deposition commenced. This method should no tbe used if the basis metal or undercoat is exposed or is likely to become exposed during anodic treatment h the electroplating bath.
Copyright(C) The Hard Chromium Platers Association of Japan