Read the Rust Stains

Posted: May 1, 2012

Spotting a leak in the exhaust ­system riser leads to an early fix.

By: Deane Hislop

Easy Goin’ was on the hard at Cap Sante Marine in Anacortes, for some regularly scheduled spring maintenance. Barney, the service mechanic, and I were in the engine room inspecting the drive­train when Barney said, “What’s this?” It was a small brown stain, the size of a nickel, on the port stringer. Before I could say “I don’t know,” Barney was deep into looking for the source and pulling the insulation blanket off the exhaust riser.

“There it is,” Barney said. The stainless steel riser had a hairline crack and an area of corrosion. The riser had been leaking salt water, the blanket was capturing the liquid and the heat of the exhaust created a perfect environment for corrosion and potential catastrophic failure down the road. Stainless steel is prone to a phenomenon known as crevice corrosion, particularly when it is exposed to high temperatures and/or when used in a manner that traps stagnant water when the system is idle. The brown stain that Barney found on the stringer was a textbook sign of crevice corrosion.

The marine wet-exhaust system is a torture chamber if ever there was one. The temperature of the exhaust can exceed 800 degrees Fahrenheit, and it mixes with water that is obviously much cooler, creating a rapid change in temperature and contraction of the gases. The temperature differential on the metallic portion of the exhaust can be several hundred degrees across just a few inches, creating incredible stress that is cycled hundreds and thousands of times over the life of the system components.

A variety of materials have been used for water-cooled exhaust systems, from ordinary steel and cast iron to stainless steel and more exotic corrosion-resistant alloys such as cupro­nickel and Inconel. Different materials offer different advantages and liabilities. Ordinary steel is strong, inexpensive and easily fabricated; however, it is highly prone to corrosion, which is accelerated by high temperatures. Stainless steel is significantly more corrosion resistant than ordinary steel; however, it is also more expensive, requires different welding skills and equipment (it benefits from MIG and TIG welding), and, contrary to popular belief, it’s not completely immune to corrosion and failure.

Cupronickel is even more corrosion resistant than stainless, and Inconel goes a step further in its corrosion-resistance properties — it’s used on ships and submarines in their nuclear power- and nuclear propulsion-plant cooling systems.

To complicate matters, stainless steel is available in a variety of alloys, some of which are significantly more corrosion resistant than others. Ideally, if stainless steel is used for a wet-exhaust system, it should use an alloy designated as 316L. This alloy is especially corrosion resistant, and the all-important “L” suffix indicates it has a low carbon content that makes it well suited to welded fabrication. Other stainless alloys, such as 304, while identical in appearance to 316, should not be used for highly corrosion-prone applications, particularly exhaust systems.

Wet metallic exhaust systems often face an additional challenge: adequate support. Because they are often large, complex, heavy and subject to extreme vibration, metallic exhaust systems are doubly prone to stress-induced corrosion and failure. When stainless steel is stressed, microscopic cracks form on its surface, each of which becomes a toehold for corrosion. Thus, metallic exhaust systems must be well supported to minimize as much movement as possible.

Inspect your exhaust system regularly for signs of this corrosion and potentially catastrophic failure.

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