Marine Exhaust Materials & Available Coating Options
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Topic: Marine Exhaust System Materials & Coating’s
Question #1: What is the BENEFIT of Aluminum versus Cast Iron with regards to my Marine Exhaust System?
Question #2: What is the purpose of Coating a Marine Exhaust system?
Response #1: The main advantage of Aluminum versus Cast Iron is with regards the weight. Aluminum has approximately 33% of the density of Cast Iron and therefore will weigh about 1/3 of the weight of the same part in Iron. Many people think there is a big benefit with regards to corrosion but here’s our best explanation in Layman’s terms. When you think of Outboard engines versus Sterndrive’s which one do you suppose does best with regards to resisting corrosion? Answer – Outboards. Why? Because they made out of Aluminum and not left in the salt water when not in useage. Aluminum has the unique characteristic that it reacts with Oxygen and therefore creates a protective layer that resists corrosion. This is the white, chaulky powder you see on Aluminum that is exposed to the elements. This is also one of the reasons big inboard boats are normally equipped straight shafts versus sterndrives that are exposed to corrosive water. Outboards do well because they’re out of the water and able to react with the air and therefore generate that protective layer, whereas a sterndrive cannot be completely removed from the water unless the boat is placed on a lift. As for the exhaust system, coolant water normally remains in the manifold and therefore causes a problem for an aluminum manifold in not being exposed to the air, however the mode of corrosion for Cast Iron is exactly the opposite where it is better to leave it submerged in water where it is not exposed to air and therefore cannot rust. This is the same reason the exhaust riser/elbow tends to corrode worse than the manifold since the riser never has water remaining in it after the engine is shut down like the manifold. Therefore if you are going to select an Aluminum exhaust system it should be a cermic coated aluminum product otherwise you are better sticking with cast iron. The best option for corrosion resistance is teh infused ceramic coated aluminum product.
Response #2:
The two primary failure modes with marine exhaust manifolds are associated with either corrosion related failure, or freeze damage. Regarding freeze damage one must always ensure the cooling water is drained before the onset of freezing weather. Regarding corrosion there are a couple of different methods of controlling the life of the parts.
Alternative Materials – This is a very good option however very costly and really not available in most cases. Standard stainless steel systems generally sell for about 3 times the cost of cast iron or aluminum.
Coatings – Coatings are a very cost effective method of extending the life expectancy of iron or aluminum marine exhaust manifolds. There are several different coatings available ranging from galvanizing, to Ceramic Coating for Aluminum, to electro deposition plating (EDP). Since the determining factor regarding the life expectancy is not only how quickly the coating CORRODES but also how quickly the coating ERODES. Therefore a coating thet resists errosion has significant benefit versus coatings that simply rely on adhesion. In our experience the best combination for a coating strategy is with the following approach;
- Cast Iron Ceramic Coating (sacrificial). When most people think of exhaust system ceramic coatings they think it has something to do with a heat barrier of some sort. However for marine exhaust systems the heat issue is not that important because there is an abundance of coolant water. The purpose is 100% for corrosion resistance. Since all coatings are porous some amount of the corrosive salt water will penetrate the coating and attack the base material. Therefore a sacrificial coating ensures that any corrosion that takes place does so to the coating before the actual base iron or aluminum material. In a nutshell this allows the coating to function as specified; extending the life of the casting.
- Aluminum Ceramic Coating – One of the best of the coating options because this coating actually fuses itself into the micro structure of the Aluminum therefore creating a semi non-penetrable corrosion barrier.
- Electro Deposition Plating – This acts as a protective outside barrier to further extend the life of the casting by further preventing salt water from penetrating the ceramic coating. The fact that the EDP coating offers complete internal and external coverage is a huge benefit. The double EDP coated products as we supply actually have 2 different layer/coats and therefore work in conjunction to offer the most complete coverage available. This is the most economical option available for Cast Iron or Aluminum systems, and when comparing cost versus performance typically yields the highest overall value.
In all cases when installing the system you do not need to use silicone type gasket sealers. The most important things to ensure when installing marine exhaust manifolds and risers is to make sure the fasteners are torque consistently such that the gaskets is compressed equally (criss-cross pattern). Torque the fasteners in 3 separate stages, the first just to “seat” the gaskets, then followed by the next two in equal amounts of half the total required torque. For example if the total required torque is 50 ft – lbs the first step would be to seat the gasket by tightening the fasteners to about 5 ft-lbs (criss-cross pattern). The next step would be to torque the fasteners to about 25 ft-lbs (criss-cross pattern), while the third step would be achieving the final specified torque in this case 50 ft-lbs (criss-cross pattern). After running the marine engine and allowing the cooling system to reach operating temperature, the marine exhaust manifold and riser fasteners should be re-torque’d to the specified torque. This ensures that good fastener retention is maintained after the marine engine completes the post installation thermal cycle.
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