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Hard chromium plating
Technical Report Hard Chromium Plating
Finnish company Cromax Oy has been using reduced pressure plating techniques since 19 76. This report looks at how and why it is claimed that the quality of hard chromium deposits produced in this way is enhanced. Hard chromium plating is one of the most difficult electroplating processes. The quality of the coating is always influenced by hydrogen evolution occurring on the cathode surface simultaneously with chromium deposition. Hydrogen bubbles tend to remain on the surface and give rise to pores in the deposited chromium layer. Defects in the coating decrease locally the corrosion resistance of the layer. The decomposition of chromium hydrides causes the formation of a crack network in the coating. Gas formation causes splashing of the electrolyte. This means problems in occupational safety when handling hexavalent chromium solutions. The reduced pressure plating method involves electroplating in partial vacuum, about 40kPa below atmospheric pressure. It is carried out either using suction equipment connected directly to a cylindrical object, or by means of a process chamber (Fig 1). Here, the electrolyte circu1. Process chamber 2. Object to be pleted 3. Cover with seals 4. Electric power connection to object to be 5. Pressure gauges 6. Check valves 7. Inlet chamber o f circulation sluice device 8. Vacuum pump 9~ Parallel inlet chamber o f sluice device 10. Outlet chambers o f sluice device 1 I. Return flow tubing 12. Cover with lowered portion (can be used, for small objects requiring internal platinj 13. Control panel 14. Temperature regulated electrolyte reserw
lates through the process chamber under reduced pressure. The overall quality of the coating is considerably improved. When the pressure is lowered about 15kPa below atmospheric pressure, the mist vapours developed by the process are absorbed by the elctrolyte. The method is especially suitable for metal objects with high demands on coating quality. For instance, plating of machine parts for the shipbuilding, plastic, paint and printing industries, plating of cutting and forming tools and inside hard-chromium plating of firearm barrels are suitable applications. Inside chromium plating of tubes and other hollow sections is especially easy to perform. The object itself can be used as a process chamber by means of connecting the inlet and outlet of the electrolyte circulation system directly to the object. Any conventional chroming electtrolyte can be used.
5.
5
Fig. 1 Flow scheme of the hard chromium plating system working with the reduced pressure method.
126
MATERIALS & DESIGN Vol. 5 JUNE/JULY 1984
Coating Characteristics Gaseous hydrogen is effectively released from the base metal and from the coating, which considerably reduces the porosity of the coating. The coating will smooth up a rough or uneven surface due to the more effective deposition process. The quality of the coating is improved because the chromium grain size is small. The electrolyte remains homogeneous due to its effective circulation. Since no gas bubbles are trapped under the coating, adhesion to the base metal is improved. The requirements on quality and nature of the base metal are less strict even cast-iron can be successfully chromium plated without special preparation. Reduced pressure plating reduces health hazards and also generally improves working conditions. The use of a closed vacuum and electrolyte circulating system eliminates the risk of having toxic gases and vapours in the working environment; the closed system also eliminates electrolyte splash. Crack Network In several samples of hard chromium coating made by the reduced pressure plating method, no crack network could be observed with an optical stereomicroscope, magnification 100x. Only in a scanning electron microscope (SEM) could some cracks be observed. An SEM micrograph also showed the very fine grains in the chromium coating and that porosity was absent. Corrosion tests failed to reveal pores, since they do not reach the whole coating to the base metal.
Chromium plating on cast iron adheres very well to graphite microflakes. A surface with V-formed grooves was coated to a thickness of 40MM without there being any substantial difference between the prof'des of the coated and uncoated materials; the object was examined by the Metal Laboratory of the Technical Research Centre of Finland using a Talysurf meter. Application Notes l~'ston r o d , the piston rod to be plated is positioned cent-
rally within a cylindrical anode and plating is carried out in a closed chamber. Several piston rods can be plated simultaneously. Cylinders - the method uses the cylinder itself as the process container. The anode is inserted along the axis of the cylinder and the electrolyte is pumped through the cylinder under sub-atmospheric pressure. The flow of electrolyte carries away the gases evolved, so that the surface of the metal is constantly bathed by fresh electrolyte at a controlled temperature. The method is suitable for both blind cylinders and through cylinders. Forging and moulding tools - the parts of the tools form a closed chamber with the plating anode. Deep blind holes and sharp internal corners are difficult to plate, although the method is suitable for plating extraction fingers, extraction rings and injection nozzles. Diesel engine piston rings grooves - a specialised tool is used that surrounds the pston and positions the anodes correctly in relation to the piston ring grooves. The tool is so designed that a uniform flow of electrolyte is obtained
f(%)
y.'/.
i
24 I
~0 t " - - ' a
22
.a
....
-
fluoride
bath
~--"
I
;o
sulphate
20
bath
(3"-
18 ~0
16 ?0 SRHS-bath
lz, I0 1"
I
I
0 (0
20 32
40 7,0
!
60 10,0
.
z~p. kPa v. I/min,
Fig. 2 Current efficiency o f different plating baths as function of pressure reduction (o - sulphate bath, X - fluroride bath, - SHITS bath).
MATERIALS& DESIGN Vol. 5 JUNE/JULY 1984
0 (0
20 32
40 7.0
60 I0.0
,~p. kPa v. I / m i n )
Fig. 3 Average porosity (f,%) as function of pressure reduction p and flow rate of bath, v.
127
past all surfaces to be plated. The surface of the plating is uniform and thickness can be controlled very exactly. This reduces the amount of subsequent machining and finish grifiding may not be necessary. The method is also suitable for very large pistons that can be difficult to lift or rotate during plating. Costs should be about the same, or less than, those of conventional methods but the quality and finish should be better. Journal bearing surfaces on complicated shafts - a tool is made that carries the anodes and positions them concentrically round the bearing surfaces to be plated, as well as providing for supply and discharge of the electrolyte and the necessary liquid sealing. The tool can be used on a large number of shafts. The method allows complicated shafts to be hard chromed without time-consuming masking. The chrome layer is smooth and tolerances are close. Final grinding is easier and can sometimes be omitted entirely or replaced by belt polishing. Barrel plating - the vacuum method improves the quality and yield of barrel plating methods. The risk of leakage is reduced and working conditions are improved. The method is also suitable for other surface treatment carried out by the barrel method and the same benefits should be obtained.
Conclusions Experiments carried out by the Technical Research Centre of Finland have shown that the reduced pressure plating method has the following advantages for hard chromium plating. 1 The current efficiency of the chromium electrolysis is increased - with the same current in the same time 5-10% thicker coatings can be deposited. 2 The porosity of chromium coatings is decreased. Depending upon plating conditions it can be as low as one tenth of the porosity of coatings conventionally deposited.
3 The roughness of the coatings is decreased about 1 0 20%, depending on plating conditions. The reduced pressure, method does not in itself guarantee a good result in hard chromium plating, but it is an important factor. Cromax Oy has applied for patents, which have been granted in several countries.
5O ~0 50 O0 5G
0 (0
1
I
20 3,2
40 7.0
I , ~...
60 10,0
~p, kF~ v. /~rain)
Fig. 4 Average time for rust appearance in the NSS (neutral salt spray) test for samples plated in the SRHS bath.
Ra
Sample
alues
L~p = 0
%p = 40 kPa
unplated steel
0.13
0.13
unplated s t e e l , anodic
0.17
0.17
0.41
0.37
0.50
0.41
0.44
0.35
etching 15 s, 70 A/dm2 50 /um of chromium, sulphate bath 50/um of chromium, #
fluoride
bath
50 /um of chromium, SRHS-bath
Table 1 Average roughness (Ra) values for base metal and some chromium plates (i = 50A/dm2;t = 50°C).
128
MATERIALS & DESIGN Vol. 5 JUNE/JULY 1984
Finnish company Cromax Oy has been using reduced pressure plating techniques since 19 76. This report looks at how and why it is claimed that the quality of hard chromium deposits produced in this way is enhanced. Hard chromium plating is one of the most difficult electroplating processes. The quality of the coating is always influenced by hydrogen evolution occurring on the cathode surface simultaneously with chromium deposition. Hydrogen bubbles tend to remain on the surface and give rise to pores in the deposited chromium layer. Defects in the coating decrease locally the corrosion resistance of the layer. The decomposition of chromium hydrides causes the formation of a crack network in the coating. Gas formation causes splashing of the electrolyte. This means problems in occupational safety when handling hexavalent chromium solutions. The reduced pressure plating method involves electroplating in partial vacuum, about 40kPa below atmospheric pressure. It is carried out either using suction equipment connected directly to a cylindrical object, or by means of a process chamber (Fig 1). Here, the electrolyte circu1. Process chamber 2. Object to be pleted 3. Cover with seals 4. Electric power connection to object to be 5. Pressure gauges 6. Check valves 7. Inlet chamber o f circulation sluice device 8. Vacuum pump 9~ Parallel inlet chamber o f sluice device 10. Outlet chambers o f sluice device 1 I. Return flow tubing 12. Cover with lowered portion (can be used, for small objects requiring internal platinj 13. Control panel 14. Temperature regulated electrolyte reserw
lates through the process chamber under reduced pressure. The overall quality of the coating is considerably improved. When the pressure is lowered about 15kPa below atmospheric pressure, the mist vapours developed by the process are absorbed by the elctrolyte. The method is especially suitable for metal objects with high demands on coating quality. For instance, plating of machine parts for the shipbuilding, plastic, paint and printing industries, plating of cutting and forming tools and inside hard-chromium plating of firearm barrels are suitable applications. Inside chromium plating of tubes and other hollow sections is especially easy to perform. The object itself can be used as a process chamber by means of connecting the inlet and outlet of the electrolyte circulation system directly to the object. Any conventional chroming electtrolyte can be used.
5.
5
Fig. 1 Flow scheme of the hard chromium plating system working with the reduced pressure method.
126
MATERIALS & DESIGN Vol. 5 JUNE/JULY 1984
Coating Characteristics Gaseous hydrogen is effectively released from the base metal and from the coating, which considerably reduces the porosity of the coating. The coating will smooth up a rough or uneven surface due to the more effective deposition process. The quality of the coating is improved because the chromium grain size is small. The electrolyte remains homogeneous due to its effective circulation. Since no gas bubbles are trapped under the coating, adhesion to the base metal is improved. The requirements on quality and nature of the base metal are less strict even cast-iron can be successfully chromium plated without special preparation. Reduced pressure plating reduces health hazards and also generally improves working conditions. The use of a closed vacuum and electrolyte circulating system eliminates the risk of having toxic gases and vapours in the working environment; the closed system also eliminates electrolyte splash. Crack Network In several samples of hard chromium coating made by the reduced pressure plating method, no crack network could be observed with an optical stereomicroscope, magnification 100x. Only in a scanning electron microscope (SEM) could some cracks be observed. An SEM micrograph also showed the very fine grains in the chromium coating and that porosity was absent. Corrosion tests failed to reveal pores, since they do not reach the whole coating to the base metal.
Chromium plating on cast iron adheres very well to graphite microflakes. A surface with V-formed grooves was coated to a thickness of 40MM without there being any substantial difference between the prof'des of the coated and uncoated materials; the object was examined by the Metal Laboratory of the Technical Research Centre of Finland using a Talysurf meter. Application Notes l~'ston r o d , the piston rod to be plated is positioned cent-
rally within a cylindrical anode and plating is carried out in a closed chamber. Several piston rods can be plated simultaneously. Cylinders - the method uses the cylinder itself as the process container. The anode is inserted along the axis of the cylinder and the electrolyte is pumped through the cylinder under sub-atmospheric pressure. The flow of electrolyte carries away the gases evolved, so that the surface of the metal is constantly bathed by fresh electrolyte at a controlled temperature. The method is suitable for both blind cylinders and through cylinders. Forging and moulding tools - the parts of the tools form a closed chamber with the plating anode. Deep blind holes and sharp internal corners are difficult to plate, although the method is suitable for plating extraction fingers, extraction rings and injection nozzles. Diesel engine piston rings grooves - a specialised tool is used that surrounds the pston and positions the anodes correctly in relation to the piston ring grooves. The tool is so designed that a uniform flow of electrolyte is obtained
f(%)
y.'/.
i
24 I
~0 t " - - ' a
22
.a
....
-
fluoride
bath
~--"
I
;o
sulphate
20
bath
(3"-
18 ~0
16 ?0 SRHS-bath
lz, I0 1"
I
I
0 (0
20 32
40 7,0
!
60 10,0
.
z~p. kPa v. I/min,
Fig. 2 Current efficiency o f different plating baths as function of pressure reduction (o - sulphate bath, X - fluroride bath, - SHITS bath).
MATERIALS& DESIGN Vol. 5 JUNE/JULY 1984
0 (0
20 32
40 7.0
60 I0.0
,~p. kPa v. I / m i n )
Fig. 3 Average porosity (f,%) as function of pressure reduction p and flow rate of bath, v.
127
past all surfaces to be plated. The surface of the plating is uniform and thickness can be controlled very exactly. This reduces the amount of subsequent machining and finish grifiding may not be necessary. The method is also suitable for very large pistons that can be difficult to lift or rotate during plating. Costs should be about the same, or less than, those of conventional methods but the quality and finish should be better. Journal bearing surfaces on complicated shafts - a tool is made that carries the anodes and positions them concentrically round the bearing surfaces to be plated, as well as providing for supply and discharge of the electrolyte and the necessary liquid sealing. The tool can be used on a large number of shafts. The method allows complicated shafts to be hard chromed without time-consuming masking. The chrome layer is smooth and tolerances are close. Final grinding is easier and can sometimes be omitted entirely or replaced by belt polishing. Barrel plating - the vacuum method improves the quality and yield of barrel plating methods. The risk of leakage is reduced and working conditions are improved. The method is also suitable for other surface treatment carried out by the barrel method and the same benefits should be obtained.
Conclusions Experiments carried out by the Technical Research Centre of Finland have shown that the reduced pressure plating method has the following advantages for hard chromium plating. 1 The current efficiency of the chromium electrolysis is increased - with the same current in the same time 5-10% thicker coatings can be deposited. 2 The porosity of chromium coatings is decreased. Depending upon plating conditions it can be as low as one tenth of the porosity of coatings conventionally deposited.
3 The roughness of the coatings is decreased about 1 0 20%, depending on plating conditions. The reduced pressure, method does not in itself guarantee a good result in hard chromium plating, but it is an important factor. Cromax Oy has applied for patents, which have been granted in several countries.
5O ~0 50 O0 5G
0 (0
1
I
20 3,2
40 7.0
I , ~...
60 10,0
~p, kF~ v. /~rain)
Fig. 4 Average time for rust appearance in the NSS (neutral salt spray) test for samples plated in the SRHS bath.
Ra
Sample
alues
L~p = 0
%p = 40 kPa
unplated steel
0.13
0.13
unplated s t e e l , anodic
0.17
0.17
0.41
0.37
0.50
0.41
0.44
0.35
etching 15 s, 70 A/dm2 50 /um of chromium, sulphate bath 50/um of chromium, #
fluoride
bath
50 /um of chromium, SRHS-bath
Table 1 Average roughness (Ra) values for base metal and some chromium plates (i = 50A/dm2;t = 50°C).
128
MATERIALS & DESIGN Vol. 5 JUNE/JULY 1984