How to Join Two Materials That Do Not Like Each Other

Why this matters

You do not usually get to choose both sides of a joint. The building has galvanized steel and your new work is copper. The rooftop stand is galvanized and the rail is aluminum. The existing riser is metal and the replacement run is plastic. Refusing the pairing is not on the table, so the skill is making the connection in a way that survives, and the method is decided by the joint's geometry more than by the two materials.

Getting it wrong does not fail at install. It fails in a season or in five years, at one point, and the next contractor replaces the part rather than the pairing, which starts the clock again.

The two incompatibilities, because the fixes can fight each other

Electrochemical. Two different metals in metallic contact with an electrolyte bridging them form a cell, and the more active one gives itself up. Three conditions are required and removing any one stops it. The pairing rules, the galvanic ordering and the area-ratio multiplier are covered in depth in the siblings listed at the end; the one line to carry into this article is that the small part should be the noble one, because a small active part beside a large noble one concentrates all the loss into the small part and it goes fast.

Dimensional. Two materials at the same temperature change move by different amounts. Restrain them against each other and the difference turns into stress, which comes out as a cracked fitting, a sheared fastener, a buckled run or a joint that pumps itself loose over thousands of cycles.

These two demands can conflict on the same joint. An isolating sleeve solves the electrical half and can restrain the mechanical half if you install it into a hole with no clearance. Handle both, in that order, at every joint below.

Before you break in

On a pressurized or fluid-filled joint: close and lock the isolating valves, apply lock and tag to each energy isolating device, drain or vent, and verify zero pressure at a gauge or an open vent on the isolated side before loosening the first connection, which is what 29 CFR 1910.147 requires for isolating and verifying stored energy in general industry. Let hot lines cool to hand temperature before breaking them.

One rule specific to this work: metal water piping in a building is commonly part of the required electrical bonding system, and you do not create galvanic isolation by interrupting it. Where an isolating fitting breaks metallic continuity in piping that is bonded, the bonding path is restored with a listed bonding jumper across the fitting, keeping the bonding required by NFPA 70 (the National Electrical Code) Article 250 intact. What that means for your corrosion strategy is the subject of a companion article listed in the references; the instruction here is simply that removing a bonding conductor is never part of the fix.

Joint type 1: the threaded pipe transition

The most common dissimilar-metal joint in the trade, and the one where the details are most often skipped.

• Use a purpose-made transition. A dielectric fitting, or a section of a third material such as a length of brass or a proper transition nipple, gives you a defined interface rather than one metal cut into the other.
• Get the flow direction right. In a flowing water line, put the more noble metal downstream. Copper releases ions into the water, and those ions deposit on more active metal downstream and pit it. Copper upstream of galvanized steel is a slow attack on the steel that no fitting at the joint prevents, because the mechanism travels in the water rather than through the joint.
• Match the sealant to the service and to the fitting. A conductive joint compound smeared across an isolating fitting can bridge what the fitting was installed to separate.
• Do not rely on thread engagement being dry. Threads hold water by capillary action. The crevice inside a threaded joint is a preferred site for corrosion whether the metals match or not.

Joint type 2: the flanged or bolted transition

Here you have more control, and more ways to leave the isolation half-done.

A complete isolation kit is a gasket, a sleeve for each bolt, and a washer set on both sides. Missing any one of those leaves a metallic path, and the most common miss is washers on one side only, which looks complete from the side you are standing on.

Order of assembly matters: sleeve into the bolt hole first, then the washer stack, then torque, and check that the sleeve has not been shaved or bunched by the bolt going in. A sleeve cut by an installation is a sleeve that is no longer isolating.

Then verify rather than assume. With the joint assembled and the system out of service, put a meter across the two flanges and confirm you read open rather than continuity. If the joint is on bonded piping, do this before you install the bonding jumper, or you will be measuring the jumper.

Joint type 3: the mechanically fastened bracket, rail or panel

Structural pairings on rooftops, exterior walls and equipment stands. Water is intermittent here rather than continuous, which changes the strategy: your best tool is drainage, not sealant.

• Choose the fastener as the small noble part. A stainless fastener through an aluminum rail is a small noble item in a large active one, which is the geometry you want. An aluminum fastener holding a large stainless assembly is the same pairing with the ratio inverted, and it fails.
• Isolate the faying surface with an isolating washer, a polymer tape or a compatible barrier between the two materials, sized to cover the whole contact area rather than just the fastener seat.
• Do not create a water trap. A joint sealed on the top and open at the bottom drains. A joint sealed all the way around traps whatever gets in through the fastener hole and holds it against both metals permanently, which is worse than leaving it open.
• Cut edges and drilled holes lose their coating. A galvanized member drilled on site is bare steel inside that hole. Treat the cut edge with a compatible cold-galvanizing or coating product before assembly.

Joint type 4: the bonded joint

Adhesive bonding between dissimilar materials removes the metallic path by definition, but it introduces two new failure routes.

Surface preparation is the whole job: clean to the manufacturer's stated method, abrade if the datasheet calls for it, and respect the stated open time between prep and bond. A bond made on a contaminated surface tests fine at install and peels later.

Then check the adhesive's own movement tolerance against the differential expansion of the two adherends. A rigid adhesive across two materials that move differently is a stress concentrator at the bond line; a flexible one accommodates the movement and is often the correct choice even where its raw strength is lower.

Joint type 5: the joint that has to move

Where the two materials differ substantially in thermal expansion, the joining method has to give the difference somewhere to go. Common thermoplastics expand several times as much as copper or steel for the same temperature change, so a plastic run anchored hard at both ends to metal is the standard version of this failure.

The moves available, in order of how often they are the right one: leave the run free to move with sliding supports and anchor it at one point only; put in an expansion loop or offset sized from the manufacturer's own chart; use slotted holes at brackets, with the fastener snug rather than clamped tight; or fit a purpose-made expansion joint where the layout cannot absorb the movement.

Worked example: a rooftop stand and the run on top of it

A shop replaced a rooftop condensate and piping arrangement on a galvanized steel stand, adding an aluminum rail and a thermoplastic drain run.

The rail joint. Aluminum rail onto galvanized steel, outdoors, wet whenever it rains, which puts all three galvanic conditions on the table. The fastener call was stainless through the aluminum rail, which is a small noble part in a large active one, so the loss is spread across a large area rather than concentrated. Between rail and steel went an isolating strip covering the full contact area. The drilled holes in the galvanized member were treated at the cut before assembly, because a drilled hole in galvanizing is bare steel and the hole is exactly where water sits. The assembly was left open at the bottom so it drains rather than sealed all round.

The alternative that had been on the truck, aluminum fasteners with a bead of sealant around each, would have inverted the area ratio and trapped water in the joint. It would have looked identical on the day and been the failure point in a couple of seasons.

The drain run. The run is about 60 feet of thermoplastic on the roof, seeing roughly a 60 degree F swing between a winter night and a summer afternoon. The manufacturer's expansion table for this material gives, illustratively, around 0.4 inch of movement per 100 feet per 10 degrees F, and your own material's table will differ, so pull it rather than borrowing this number.

Working it through: 60 feet is 0.6 of 100 feet, so about 0.24 inch per 10 degrees F. Across a 60 degree F swing, that is six of those increments, or about 1.44 inches of total movement in the run.

An inch and a half is not a rounding error. Anchored rigidly at both ends to the metal stand, that movement goes into the joints and the anchors as force. So the run was anchored at one point near the middle, supported on sliding hangers elsewhere so it can grow in both directions, and given an offset near the far end sized from the manufacturer's chart for the calculated movement. Brackets used slotted holes with the fastener snug, not clamped.

The failure mode this avoids is specific and recognizable: cracked fittings at the anchored ends, appearing after the first full seasonal cycle rather than at commissioning, and usually diagnosed as a bad fitting. Two or three fittings replaced over two years in a run that is still rigidly anchored is that failure, being treated as a parts problem.

The checks before you call the joint done

• Continuity across an isolating joint. Meter across it, out of service, expecting open. Do this before installing any bonding jumper so you are measuring the isolation, not the jumper.
• Bonding intact. Confirm the required bonding path is present and continuous around any isolating fitting you installed in bonded piping. Isolation and bonding are both required; one does not substitute for the other.
• Movement available. Push the run by hand at a slider and confirm it moves. A support that was tightened down as a habit has converted a slider into an anchor, and this is the single most common way a correctly designed expansion arrangement gets defeated during installation.
• Water can leave. Look at the finished joint and find the path water takes out of it. If there is not one, you have built a reservoir between two metals.

References

• 29 CFR 1910.147, control of hazardous energy: isolation, stored energy relief and verification before servicing
• NFPA 70 (National Electrical Code), Article 250, bonding of metal water piping and equipment
• Manufacturer expansion tables for coefficient of thermal expansion, loop and offset sizing, and adhesive surface preparation and movement capability
• See related: Galvanic Corrosion and the Metals That Fight; The Galvanic Series as a Field Tool; The Fastener That Was the Wrong Material; The Dielectric Union and What It Does Not Fix