Magnesium Finishing - Part 11 -

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– Part 11 – Plating / Continued from Galvanotechnik 12/2024

Magnesium is categorised as a difficult metal for plating, because of its high susceptibility to corrosion in aqueous solutions. It reacts with atmospheric air and water, resulting in the formation of an instant oxide-carbonate film on the surface. The presence of this oxidation film prevents the formation of good bonding of subsequent coatings on the substrate. In this article the process sequence for successful electro- and electroless plating on magnesium alloys is described.

The most important part of plating of magnesium alloys is often the selection of an appropriate pre-treatment, and deposition cycle. The pre-treatment should promote a relatively rapid initiation of the coating and thereby mini­mize the prior formation of magnesium oxide/ hydroxide at the surface of the substrate. After surface cleaning in alkaline solutions followed by etching in chromates and fluorides, the magnesium alloys can be successfully coated with an immersion zinc layer. Though this zinc layer is too thin, it enables successful plating of compact layers of Zn, Cu, Ni, binary, ternary alloys. Progress of plating on mag­nesium alloys is reviewed elsewhere [1-3]. Some typical plating baths are depicted in Table 1. Electroless nickel- and gold-plated magnesium parts are shown in Figure 1.

 Tab. 1: Some electrolytic and electroless plating baths [1,3]

Coating

Bath formulations and operating conditions

Zinc electroplating

Zn2P2O7: 21 g/L, K4P2O7.10H2O: 260 g/L, C6H17O7N3: 25 g/L, CS(NH2)2: 1-2 g/L, and brightener: 0.02-0.05 g/L, operating at pH: 9.0-10.5, 40-85 ºC. 2-3 A/dm2, ~10 min.

Copper electroplating

Cu2P2O7: 60 g/L, K4P2O7.3H2O: 300 g/L, (NH4)3C6H5O7: 25 g/L; 45 °C, 1.5 A/dm2, 20 min.

Nickel electroplating

NiSO4.6H2O: 110-130 g/L, F-: 1.0-1.5 mol/L, and buffer agent; pH: 4.8-5.4, 45-55 °C, 3.0 A/dm2, 20 min.

Electroless copper plating

CuSO4.5H2O: 6 g/L, NaH2PO2.H2O: 15 g/L, Na3C6H5O7: 28 g/L, NiSO4.6H2O: 0.5 g/L, H3BO3: 30 g/L, stabilizer: 0.2 mg/L, pH: 9.2, 65 °C for 90 min.

Fig. 1: a) electroless nickel, and b) gold plated magnesium parts (Photo: Amand Sharma)

The details of pre-treatment, Zn-immersion, and electroless-Ni processes are discussed in the subsequent sections. A passivation treatment after final electroless Ni-P in a solution of 2.5 g/L CrO3, and 120 g/L K2Cr2O7, at 90-100 °C for ~10 min. is recommended for enhancement of corro­sion protection [1].

Electroless Ni-P/ Gold plating

Electroless process has excellent throwing power providing uniform coverage. Alloying nickel with phosphorus or boron brings significant improvement in mechanical properties and corrosion resistance. Gold has high resistance to environmental corrosion, excellent electrical and thermal conductivity, exceptional solderability and superb reflectivity for infrared radiation. The combination of these properties has made gold coating as an exceptional choice for numerous engineering applications.

A process of gold plating on Mg alloy RZ5, comprising the following sequences was reported by Rajagopal et al. [4,5].

  1. Acetone degreasing
  2. Alkaline cleaning
  3. Mild chromic acid pickling (to remove the surface oxide, loosely adherent cold worked metal and surface contamination)
  4. Fluoric acid dip: HF (48%) 7.3 vol.%, 25-30 °C, 10 min.
  5. AC electrolytic treatment at 5 V for two min. in above electrolyte. This step is very important to ensure plating uniformity. It makes the whole surface of the job homogeneously active to receive the subsequent electroless nickel deposition. This is probably due to formation of an adsorbed film of fluoride after dissolving away the surface oxide and hydroxide. Omitting this step results in patchy deposition.
  6. Electroless nickel plating from a bath containing basic nickel carbonate: 10 g/L, hydrofluoric acid (70%): 9 ml/L, citric add: 5 g/L, ammonium bifluoride: 10 g/L, sodium hypophosphite: 20 g/L, ammonium hydroxide (30%): 30 ml/L, pH: ~7.0, 30-35 °C. A coating thickness of 5 µm was build up.
  7. Gold plating from neutral citrate bath: potassium gold cyanide: 20 g/L, ammonium citrate: 75 g/L, pH: 6, 25-30 °C, 0.25 A/dm2, 38 min. A 5-µm gold coating was deposited.

Ambat, and Zhou [6] modified the above electroless Ni-P bath by addition of thiourea, and mercapto-benzo-thiazole (MBT). The presence of MBT enhanced the plating rate, while thiourea provided the bath stability. The electroless deposits on AZ91D alloy showed amorphous structure with 7 wt.% P and a hardness value of 600-700 VHN. The process involved the following steps:

Ultrasonic degreasing in acetone → Alkaline cleaning in 10% NaOH at 60 °C for 5 min → Water rinse → Pickling in 6% chromic acid + 5% nitric acid solution for 45 sec. → Water rinse → Fluoride activation in HF (250 ml 70% HF/L) for 10 min. → Water rinse → Electroless nickel plating in a solution of basic nickel carbonate: 9.7 g/L, citric acid: 5.2 g/L, ammonium bifluoride: 7.5 g/L, hydrofluoric acid: 11 ml/L, thiourea: 0.5 mg/L, MBT: 0.25 mg/L, sodium hypophosphite: 20 g/L, ammonium hydroxide to adjust pH (7-8), temperature: 80 °C. A deposition rate of 9.33 µm/hr. was reported.

Sharma [7] developed the following process sequence that provides satisfactory gold plating on magnesium alloy AZ31B. The process has been successfully implemented on the components of different spacecraft.

  1. Solvent degreasing in isopropyl alcohol using an ultrasonic bath for 5-10 min.
  2. Alkaline cleaning in a solution containing 50 g/L sodium hydroxide and 10 g/L tri sodium orthophosphate for 6-10 min. at 55±5 °C followed by water rinsing.
  3. Acid cleaning in a solution containing chromic acid 500 g/L, for 5-10 min. at room temperature; water rinsing.
  4. Immersion zincating: ZnSO4.7H2O 50 g/L, Na4P2O7.10H2O 200 g/L, NaF 7 g/L, and Na2CO3 5 g/L, operating at pH:10.2-10.4, 80-85 ºC. Water rinsing.
    For solution preparation, zinc sulfate was first dissolved in water, then heated to 65-75 ºC. Sodium pyrophosphate was slowly added to it by stirring. While pyrophosphate was added, a white precipitate formed that later dissolved giving a clear solution. Sodium fluoride was then added to this clear solution, and finally the pH of the solution was adjusted with the addition of sodium carbonate.
  5. Electroless nickel plating [8]: NiCO3.2Ni(OH)2.4H2O 10 g/L, HF 40% 10 ml/L, C6H8O7.H2O 5.5 g/L, NH4HF2 15 g/L, NaH2PO2 20 g/L, NH4OH (25%) 30 ml/L, and NH2CSNH2 1 mg/L, operating at 85-90 °C for 40 min. Hot water dip.
  6. Heat treatment at 150 °C for 2 hr. in an electric oven with inert gas circulation.
  7. Gold plating in the bath shown in Table 2.
    Gold plating solutions were continuously filtered and agitated by cathode rod movement (6 cm/s). Platinum or platinum coated titanium anodes were used for acid and neutral gold plating baths, while for alkaline gold plating stainless steel tank itself was utilized as the anode. The anode area was kept a minimum of two times of cathode area. Water rinsing.
  8. Heat treatment at 150 °C for 2 hr. in an electric oven with inert gas circulation.
 Tab. 2: Gold plating baths

Bath composition & operating details

Gold strike

Gold plating

1. Acid Bath

Gold potassium cyanide, g/L

3-4

10-12

Citric acid, g/L

45-55

45-55

Tri sodium citrate, g/L

45-55

45-55

pH

3.7

3.6

Temperature, °C

60

60-65

Current density, A/dm2

0.15

0.30

Time, min.

2

25

2. Neutral Bath

Gold potassium cyanide, g/L

5-6

20-24

Tri ammonium citrate, g/L

70-80

70-80

pH

6.0

6.0

Temperature, °C

25-30

25-30

Current density, A/dm2

0.2

0.4

Time, min.

2

25

3. Alkaline Bath

Gold potassium cyanide, g/L

4-5

12-15

Potassium cyanide, g/L

18-20

18-20

pH

11.2

11.0

Temperature, °C

58

60

Current density, A/dm2

0.2

0.4

Time, min.

2

25

In view of the extreme reactivity of Mg-Li alloys, and the stringent specifications for space applications, two special process sequences for gold plating have been developed by Sharma [9-11]. One sequence includes the steps of zincating, autocatalytic nickel, activation, gold striking and gold plating, while the other involves the steps of direct nickel electroplating, electroless nickel and gold plating.

  1. Ultrasonic degreasing in isopropyl alcohol.
  2. Alkaline cleaning: NaOH 50-60 g/L, Na3PO4 10-12 g/L, KF 1-2 g/L at 85-90 °C for 5-10 min., followed by water rinsing.
  3. Acid cleaning for 4-7 min. in 50% chromic acid; water rinsing.
  4. Zincating: zinc sulfate monohydrate: 30 g/L, sodium pyrophosphate: 120 g/L, sodium fluoride: 5 g/L, sodium carbonate: 3-4 g/L and sodium acetate: 8-10 g/L; operating at pH: 8.0±0.5; 80-90 °C, for 5-10 min. with light agitation. Water rinsing.
  5. Electroless nickel plating for 45-60 min. (Tab. 3) followed by water rinsing.
 Tab. 3: Electrolytic and electroless nickel plating baths for Mg-Li alloys

Bath composition & operating conditions

Nickel electroplating

Electroless Ni-P plating

Basic nickel carbonate, g/L

110-130

10-12

Hydrofluoric acid, (40%), ml/L

100-115

10-11

Citric acid, g/L

35-45 g/L

5-6

Sodium dodecyl sulfate, g/L

1-2

Ammonium bifluoride, g/L

15-18

sodium hypophosphite, g/L

18-22

Ammonium hydroxide (25%), ml/L

25-35

pH

2.8-3.2

4.8-6.8

Temperature, ºC

55-60

85-90

Bath agitation and filtration

continuous

Mild, continuous

Current density, A/dm2

5.5-6.0

Time, min.

15

45-60/ 85-90

In an alternative method direct Ni-electroplating for 15 min. after acid cleaning was attempted. This was followed by electroless-Ni plating for 85-90 min. The coating thickness for electrolytic and electroless nickel was about 50 µm and 22 µm, respectively. The details of bath formulation and operating parameters for electrolytic and electroless nickel plating are given in Table 3.

  1. Activation of electroless nickel for gold plating in 10-15% sulfuric acid for 2-3 min. Water rinsing.
  2. Gold strike followed by gold plating as per Table 4. Water rinsing.
  3. Heat treatment at ~ 100 °C for 2 hr.
 Tab. 4: Gold electroplating baths for Mg-Li alloys

Bath composition & operating details

Gold strike

Gold plating

Gold potassium cyanide, g/L

3-4

10-12

Citric acid, g/L

50-60

50-60

Tri sodium citrate, g/L

50-60

50-60

pH

3-5

3-5

Temperature, °C

60-65

68-73

Current density, A/dm2

0.1-0.2

0.2-0.4

Bath agitation and filtration

Continuous

Continuous

Time, min.

3-5

30-40

Coating thickness, µm

4-6

Attempts to obtain direct electroless nickel deposits on Mg-Li alloys were not encouraging. The electroplated Ni film is porous but catalyses subsequent uniform electroless Ni-P deposition. The process of zincating, electroless nickel and gold plating is relatively simpler. Zincating acts as a good base and activator for electroless nickel plating forming uniform and adherent deposits.

The space worthiness of the coating was evaluated by the simulated environmental tests, viz, humidity (95 ± 5% at 50±1 °C for 48 hr.), thermal cycling tests (1,000 cycles of -45 °C to + 80 °C), followed by the measurement of optical properties. The gold coating obtained by providing stable infrared emissivity of the order of 0.03, confirming their extreme suitability for spacecraft thermal control applications.

Zhao, and Huang [12] described a direct electroless Ni-P process on AZ31 magnesium alloy with following sequence of operations, pre-cleaning → interlayer-8604, an organosilicon heat-resisting varnish → surface roughing → electroless deposition. The electroless Ni-P bath consisted of nickel sulphate hexahydrate: 16 g/L, sodium hypophosphite: 16 g/L, sodium pyrophosphate: 60 g/L, ammonium hydroxide (38%): 8 g/L, surfactant: 20 mg/L, stabilizer: 0.004 mg/L; pH: 9.5, 50 °C. The samples were coated for 60 min., washed with water and air-dried.

Electroless Ni-B

Wang et al. [13] reported electroless Ni-B coating on AZ91D alloy, after alkaline cleaning and acid pickling. Alkaline cleaning in Na2CO3: 20 g/L, Na3PO4·12H2O: 20 g/L, and polyoxyethylene octylphenyl ether: 10 ml/L, 75 °C, for 10 min. Water rinsing. Acid pickling in CH3COOH: 20 ml/L, and NaNO3: 40 g/L, 25 °C for 30-60 sec. This was followed by electroless Ni-B as shown in Table 5.

 Tab. 5: Electroless Ni-B baths

Bath composition & conditions

[13]

[14,15]

NiCl2·6H2O, g/L

20

Ni(CH3COO)2·4H2O, g/L

38

 

NaBH4, g/L

0.6

0.8 / 8 [15]

NH2CSNH2, mg/L

1

H2NCH2CH2NH2, ml/L

50

35

NaOH, g/L

36

110

NH4HF2, g/L

5

H3BO3, g/L

10

Toluene-p-sulfonic acid sodium salt, g/L

5

1,2-Propanediol

2

5-Sulphosalicylic acid dihydrate, mg/L

50

 

pH

13.4

>12

Temperature, °C

85

80

Plating time, min.

60 / 120 [15]

Electroless Ni-B coatings were deposited directly on AZ91D alloy by Correa et al. [14,15]. The samples were mechanically polished with wet 100 grit SiC paper, then grit-blasted with alumina (150 µm), rinsed with water, ultrasonically cleaned with ethanol, and dried in a hot air. A final cleaning was carried out in 37 g/L NaOH and 10 g/L Na3PO4 for 10 min. at 65 °C, followed by activation in 200 g/L NH4HF2 at room temperature for 10 min. The samples were then immediately transferred to the Ni-B plating solutions (Tab. 5). Scanning electron micrographs (SEM) of electroless Ni-B is shown in Figure 2.

Fig. 2: SEM of electroless Ni-B [15]Fig. 2: SEM of electroless Ni-B [15]

The processes of duplex, Ni-P/Ni-B, ternary (Ni-W-P, Ni-Sn-P), and composite electroless nickel, Ni-P-X, where X= TiO2, ZrO2, SiO2, B4C, Montmorillonite etc. were investigated to improve the hardness and the corrosion resistance of electroless coating [3,16-19].

References:

[1] N. El Mahallawy: Surface treatment of magnesium alloys by electroless Ni-P plating technique with emphasis on zinc pretreatment: A review, Key Eng. Mater., 384 (2008) 241-262. doi: 10.4028/www.scientific.net/KEM.384.241[2]L. Wu; J. Zhao; Y. Xie; Z. Yang: Progress of electroplating and electroless plating on magnesium alloy, Trans. Nonferr. Met. Soc. China, 20, Suppl. 2 (2010) s630-s637. doi: 10.1016/S1003-6326(10)60552-3
[3] A.D. Forno; M. Bestetti: Electroless and Electrochemical Deposition of Metallic Coatings on Magnesium Alloys Critical Literature Review (Chapter 8), Magnesium Alloys: Corrosion and Surface Treatments, F. Czerwinski (Editor), London: IntechOpen, (2011) 153-184. doi: 10.5772/13975
[4] I. Rajagopal; K.S. Rajam; S.R. Rajagopalan: Plating on magnesium alloy, Met. Finish., 88, no. 12 (1990) 43-47
[5] Rajagopal, I.; K.S. Rajam; S.R. Rajagopalan: Gold plating of critical components for space applications: Challenges and solutions, Gold Bull., 25, no. 2 (1992) 55-66. doi: 10.1007/bf03214720
[6] R. Ambat; W. Zhou: Electroless nickel-plating on AZ91D magnesium alloy: effect of substrate microstructure and plating parameters, Surf. Coat. Technol., 179 (2004) 124-134. doi: 10.1016/S0257-8972(03)00866-1
[7] A.K. Sharma; H. Narayanamurthy; H. Bhojaraj; J.M. Mohideen: Gold plating on magnesium alloys for space applications, Met. Finish., 91, no. 3 (1993) 34-40
[8] A.K. Sharma; M.R. Suresh; H. Bhojaraj; H. Narayanamurthy; R.P. Sahu: Electroless nickel plating on magnesium alloy, Met. Finish., 96, no. 3 (1998) 10-18. doi: 10.1016/s0026-0576(97)83005-x
[9] A.K. Sharma: Electrodeposition of gold on magnesium-lithium alloys, Met. Finish., 86, no. 12 (1988) 33-34
[10] A.K. Sharma: Gold plating on magnesium-lithium alloys, Met. Finish., vol. 89, no. 7 (1991) 16-17
[11] A.K. Sharma; H. Bhojaraj; V.K. Kaila; H. Narayanamurthy: Thermal control coatings on Mg-Li alloys, SAE Int. J. Aerosp., Sec. 1, 102 (1993) 865-872. doi: 10.4271/932123
[12] H. Zhao; Z. Huang; J. Cui: A new method for electroless Ni-P plating on AZ31 magnesium alloy, Surf. Coat. Technol., 202, no. 1 (2007) 133-139. doi: 10.1016/j.surfcoat.2007.05.001
[13] Z.C. Wang; F. Jia; L. Yu; Z.B. Qi; Y. Tang; G.L. Song: Direct electroless nickel-boron plating on AZ91D magnesium alloy, Surf. Coat. Technol., 206 (2012) 3676-3685. doi: 10.1016/j.surfcoat.2012.03.020
[14] E. Correa; A.A. Zuleta; L. Guerra; M.A. Gomez; J.G. Castano; F. Echeverria; H. Liu; P. Skeldon; G.E. Thompson: Tribological behavior of electroless Ni-B coatings on magnesium and AZ91D alloy, Wear, 305, no. 1-2 (2013) 115-123. doi: 10.1016/j.wear.2013.06.004
[15] E. Correa; A.A. Zuleta; M. Sepulveda; L. Guerra; J.G. Castano; F. Echeverria; H. Liu; P. Seldon; G.E. Thompson: Nickel-boron plating on magnesium and AZ91D alloy by a chromium-free electroless process, Surf. Coat. Technol., 206 (2012) 3088-3093. doi: 10.1016/j.surfcoat.2011.12.023
[16] W.X. Zhang; Z.H. Jiang; G.Y. Lio; J.S. Lian: Electroless Ni-P/Ni-B duplex coatings for improving the hardness and the corrosion resistance of AZ91D magnesium alloy, Appl. Surf. Sci., 254 (2008) 4949-4955. doi: 10.1016/j.apsusc.2008.01.144
[17] S. Sadreddini; Z. Salehi; H. Rassaie: Characterization of Ni-P-SiO2 nano-composite coating on magnesium, Appl. Surf. Sci., 324 (2015) 393-398. doi: 10.1016/j.apsusc.2014.10.144
[18] A. Araghi; M.H. Paydar: Electroless deposition of Ni-P-B4C composite coating on AZ91D magnesium alloy and investigation on its wear and corrosion resistance, Mater. Des., 31, no. 6 (2010) 3095-3099. doi: 10.1016 j.matdes.2009.12.042
[19] T. Miri; D. Seifzadeh; Z. Rajabalizadeh: Electroless Ni-B-MMT nanocomposite on magnesium alloy, Surf. Eng., 37, no. 9 (2021) 1194-1205. doi: 10.1080/02670844.2021.1959287

  • Ausgabe: Januar
  • Jahr: 2025
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