Special Design Measures

Two principles help quantify the potential galvanic problem:

• Anodic metals are deteriorated by galvanic corrosion
• The wider the spread, the greater the galvanic potential

The galvanic corrosion rate

The galvanic corrosion rate for a combination of metals is usually determined by laboratory tests that expose the alloys to abnormally severe conditions, typically salt-spray tests. Actual service conditions are usually much less severe, so there may be no measures required in some cases where laboratory tests indicate galvanic potential. For example:

• Salt-spray tests show that the copper content in aluminum alloy 380 makes it incompatible with any magnesium alloy in theory
• In practice, a 380 aluminum transmission housing bolted directly to an AZ91D magnesium clutch housing and subjected to road testing that included salt splash required no galvanic insulation between the members
• However, the magnesium housing required galvanic insulation from the steel bolts
• Dynacast engineers help customers determine the extent of galvanic protection required for each application

Four conditions must exist for galvanic corrosion to occur:

1. An anode, or corroding metal
2. A cathode, which is a dissimilar metal that is less anodic or more cathodic
3. An electrical contact, typically metals directly touching
4. An electrolyte, or continuous conducting liquid path

Since all four conditions must coexist, eliminating any one prevents galvanic corrosion. The galvanic circle can be broken in the design process by applying one or more of the following steps:

• Exclude or minimize the accumulation of electrolyte, usually water, at critical contact surfaces
• Choose metals with maximum compatibility
• Insulate against electrical contact

Exclude or minimize the accumulation of electrolytes

Normal measures to exclude or eliminate electrolytes:

• Use tapped blind holes with threaded studs, rather than through fastening with bolts and nuts
• Locate the assembly in areas where moisture will not accumulate – this allows the use of regular production fasteners with no galvanic insulation in most consumer electronic products, electric motors and automotive interiors
• Provide drainage, such as holes, at low points - this is common practice in automotive clutch housings

Choose alloys with maximum compatibility

Galvanic compatibility between alloys varies with their chemical compositions:

• Aluminum, zinc and ZA are mutually compatible as are magnesium, zinc and ZA
• Copper levels of 1.25 to 2.50% (maximum values) in zinc 5 and ZA alloys, which might otherwise render them incompatible with magnesium, are passivated by the presence of zinc
• Aluminum alloys 360, 392, 413, 443 and 518 with restricted copper levels are compatible with magnesium alloys
• Alloys 380, 383, 384 and 390 have higher controlled levels of copper, are not compatible, and may require galvanic protection under severe conditions
• Aluminum wrought alloys 2024, 3003 and 7075 are not compatible with magnesium die casting alloys, whereas wrought alloys in the 5000 and 6000 series are used as galvanic insulators between magnesium alloys and iron
• The position of magnesium at the top of the galvanic series indicates that galvanic corrosion is more of a problem in magnesium alloys than in aluminum, zinc or ZA

Insulate against electrical contact

Dissimilar metals with galvanic potential should be insulated from each other in environments where electrolytes such as water may be present. One or more of these methods are employed:

• Sealing compounds such as paints
• Non-absorbent tapes
• Fabricated insulators

Painting as insulation

Correctly performed, painting is a convenient means of insulation:

• Instinct may suggest that the die casting should be painted, because it is the member to be protected
• When only the die casting is painted, a small area of it transfers metal to a large area of the cathodic member through a break in the paint, and severe pitting occurs in the die casting
• When the cathodic member is painted, a very large area of the die casting transfers metal to a very small area of it
• The corrosion on the die casting is superficial and not serious. Therefore it is better to paint the cathode than the die casting; even better to paint both

Tapes as insulation

• Nonabsorbent tapes such as vinyl and rubber (as thin as 0.075mm) are suitable
• Cloth-supported tapes are not recommended; they may be counter-productive because the cloth can act as a wick
• Where practical, tapes, sealing compounds and paint coatings should extend 3.2 to 6.4 mm beyond the die casting

Fabricated insulators

Fabricated insulators may be inserted between the die casting and its fasteners or between the die casting and cathodic components:

• Washers, spacers, bushings and grommets made from or coated with compatible metals or plastic are most common
• The material choice depends primarily on operating temperature and load
• For example, metal shims and washers are used under bolt heads where heavy retention loads are applied, whereas plastic shims and washers are used with sheet metal screws
• Where very light retention is required, such as a push-on steel spring clip, closed cell sponge washers are adequate
• Open cell sponge is unacceptable because it serves as a wick, retaining water between the surfaces

Wherever possible, it is good practice to combine the insulating function with other functions to reduce the number of parts required. For example, a push-on nylon washer that retains attaching screws during assembly can be used or modified to serve as an insulator.

Creep and relaxation

The term "relaxation" indicates the loss in retention that occurs in attachments that must withstand long-term sustained loads. Relaxation can occur under conditions of sustained loading when metal temperatures are elevated sufficiently; it can occur at room temperature in zinc alloys and, to a lesser degree, in ZA alloys.

• Creep characteristics for zinc
• Creep characteristics for magnesium

The potential loss of retention at attaching points is rarely prohibitive, as the attachment usually meets functional requirements when proper design procedures are followed. Die castings are widely used in internal combustion engines, air compressors and electric motors where operating temperatures indicate potential relaxation, for example.

These guidelines, while subject to verification by testing, are a good start in designing to counteract possible relaxation effects:

1. Reduce thread stresses in the die casting substantially by increasing thread diameter; increase the length of thread engagement beyond that required to prevent stripping, and increase the number of fasteners
2. Reduce stresses where staking or swaging is used by increasing the number of staking operations and increasing the area that is swaged
3. Where possible, use through bolts and nuts rather than inserting studs or bolts in tapped holes so that compression stresses are only induced in the die casting alloy - confine shear, bending and tensile stresses to the bolt and nut
4. Use inserts where possible to distribute the loads into the die castings and reduce stress concentrations
5. Where inserts are used, design the joint so that long-term continuous loads are not transmitted to the die casting
6. Combine fastening methods, such as bonding or staking inserts in addition to interference fit

Stress corrosion cracking (AZ91 magnesium alloys only)

Magnesium alloys containing more than 1.5% aluminum may be subject to stress corrosion cracking. This category includes only the AZ series of the common magnesium die cast alloys.  Cracks may be initiated in areas where prolonged stresses of at least 30% tensile yeild (7 ksi or 48 MPa for AZ91B and D) occur in the presence of a corrosive atmosphere. Design and assembly practices that can induce continuous stresses above the limits include:

• Forcing parts into alignment for riveting or bolting
• Failure to relieve stresses after welding
• Inadequate design for press fit assemblies or cast in place inserts

Reducing residual stress

Inserts, whether cast-in-place or post-installed, induce residual stresses in the casting. To avoid stress corrosion cracking when AZ91 magnesium alloys are used, residual stresses must be limited.

• Insert wall thicknesses of 1.25mm or less will heat and subsequently contract, and limit residual stresses to safe levels
• Inserts with thicker walls must be preheated. Bearings and dowel inserts retained by interference fits may be subject to stress corrosion cracking unless residual stress levels are limited