APPLICATION TECHNOLOGY The pictures below show both cadmium and zinc-nickel
deposits with minimal white rust after more than 700 hours neutral salt spray testing.
Table 4: Galvanic corrosion potentials of plated deposits in 5% NaCl
Potential Deposit - 0.46 V - 0.5 V
- 0.76 V
Cadmium plating Zinc-nickel Zinc
Friction properties Coatings change the torque-tension properties of fasteners.
An advantage of cadmium is that it is relatively soft and has an average coefficient of friction between 0.09 and 0.14. In comparison, zinc and zinc alloy deposits have a relatively high and inconsistent friction, typically between 0.2 – 0.3. This is demonstrated in the graph below.
Contrast this with a pure zinc coating after some 120 hours. The voluminous corrosion products can be clearly seen below.
This means that a lubricant is required in order to both lower
Compatibility with aluminium 5xxx and 6xxx series aluminium alloys are used in marine
applications to take advantage of factors such as high strength to weight ratio and good corrosion resistance. However, the mechanical joining of these alloys using coated steel fasteners can lead to galvanic (bi-metallic) corrosion. It is well known that cadmium is an excellent coating choice
for steel fasteners in contact with aluminium in salt water environments, due to the small potential difference between the two metals*. In comparison, zinc is a poor choice, due to (i) the larger difference and (ii) the high corrosion rate of zinc in this environment. If we review the galvanic corrosion potentials of these deposits, we can see that zinc-nickel is very close to cadmium (Table 4). Coupled with its resistance to chloride outlined above, this means that this coating will perform similarly to cadmium in this application.
*The actual potential difference of the aluminium metal will
depend on factors such as alloy composition and whether heat treated or not.
www.macdermid.com
the coefficient of friction and make it more consistent. The lubricant is applied in the coating operation, usually as a dip following the passivation operation. If fasteners are used without a friction control fluid, the increase in friction results in lower bolt tension for a given torque, resulting in joint weakness.
Summary Passivated cadmium coatings have long been the preferred
finish for fasteners in marine applications. However, despite its excellent sacrificial corrosion resistance, compatibility with aluminium and self-lubrication, it is under pressure due to toxicity issues. Zinc-nickel with a trivalent chromium passivate is the
preferred alternative. Not only does it compare favourably with cadmium in corrosion resistance and minimal corrosion products, it is also compatible in applications where it may come into contact with aluminium. To match the coefficient of friction of cadmium coatings, a lubricant is applied. These lubricants are extensively used in today’s surface finishing industry and have the added bonus of further improving overall coating performance.
134 Fastener + Fixing Magazine • Issue 76 July 2012
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164