What materials are clevises made of?
The most common clevis materials are free-cutting steel (galvanised) and stainless steel. SYZ's clevis buyers guide describes clevis rod ends it has made in carbon steel, stainless steel, aluminium, chromoly steel and titanium. Carbon steel with a zinc finish is the default for dry service. Stainless is the step up for moisture, salt spray and washdown, and for washdown it is the 316 type to name. Many stainless clevises are made from a free-machining grade such as 1.4305 (303 type), which resists chlorides less well than 304 or 316, so ask which grade the part is (K06-Q1-08).
What does a finish do, and what happens when it is damaged?
A zinc finish protects the steel by corroding first (K06-Q1-08). If the finish is scratched or worn through, the steel underneath is exposed. Stainless steel is not immune either: chlorides, crevices and warmth can cause pitting and crevice corrosion, and 316 types resist better than 304 types.
What is galvanic corrosion?
Corrosion caused by two different metals touching in a wet place. It needs dissimilar metals in electrical contact, bridged by a conductive liquid, all three together (see BSSA on bimetallic corrosion). The less noble metal becomes the anode and corrodes, and the more noble one is protected. The three conditions are dissimilar metals, an electrolyte (the liquid) and an electrical path.
The nobility list runs from least noble to most noble: zinc, aluminium, carbon steel or cast iron, copper alloys, stainless steel, then nickel alloys and titanium. The further apart two metals are on the list, the greater the driving force. The more conductive the liquid, the greater the danger: seawater or salt-laden moist air is more of a risk than rain water or town water. And if the metals are dry, this corrosion cannot occur.
What does that mean for a clevis on a rod?
Look at the pair. An aluminium rod with a steel or stainless clevis puts aluminium, the less noble one, at risk. Galvanised steel with stainless steel is not normally considered a serious risk, except possibly in severe marine environments, where insulating barriers are usually considered adequate. That is a general statement, and the area ratio decides how it plays out: a large area of the noble metal next to a small area of the less noble one speeds the attack. A scratched coating is the textbook example of a small anode next to a large cathode (Parkway Kew).
For a clevis, the usual pair is the pin and the fork. A stainless pin in a zinc-plated fork is a small cathode on a large anode and is usually acceptable. A zinc-plated pin in a stainless fork is the reverse: a small anode on a large cathode, and the pin's coating will go quickly. If the fork is stainless, use a stainless pin.
How do you reduce the risk?
- Keep it dry, or let it dry.
- Pick metals close together on the nobility list. Material selection, insulation and coatings are the three levers.
- Insulate the two metals from each other with a barrier.
- Keep coatings whole. A scratch on a coated part is a small anode.
- Think about the liquid. Salt water is worse than rain.
What do mud and road salt do to a clevis and its bracket?
Road salt, mud and water splash, with repeated wetting and drying, are normal on a vehicle. A vehicle also tends to mix metals: an aluminium pedal box or bracket, a steel clevis, a stainless or plated pin, all in one assembly.
Mud packed into the fork is the practical problem, because it holds water against the metals for a long time and keeps the liquid that galvanic corrosion needs. Apply the nobility list above to the pair that is actually there. A steel or stainless clevis and pin in an aluminium bracket puts the aluminium, the less noble metal, at risk, especially if the joint stays wet.
The levers are the same as before. Let the joint drain and dry instead of trapping mud, keep coatings whole after stone chips and scratches, and separate aluminium from steel with a barrier such as a washer or a coating where they touch. If one metal has to corrode, make it the larger piece, which has more material to lose.
The finish and material of a safety-related pedal or steering joint are the vehicle maker's call.