What is Corrosion?
Corrosion is the degradation of materials caused by reactions between materials and environmental substances. Generally, we see it as an electrochemical reaction that is caused by atoms being oxidized on a metal surface, creating substances like rust. It’s a naturally occurring process caused by environment, as oxidized metals are in a more stable state than non-oxidized metals. Corrosion is detrimental to most metals and can cause costly destruction to pipelines, buildings, bridges, and engines.
How Corrosion Can Ruin Your Engines
As it pertains to aircraft, corrosion can happen to both active and inactive engines. Due to the intake of air that is required for most aircraft engines, this along with the exhaust gases created by the engine can cause corrosion within the engine. New engines are vulnerable to corrosion and after just a brief period of inoperativeness, corrosion can be found on the cylinder walls.
In marine engines corrosion can happen in two ways:
Hot – Vanadium and Sodium combine under high temperatures in the fuel and can corrode the exhaust passage.
Cold – Sulphur in the fuel oil can cause corrosion of the cylinder liner and combustion chamber.
Types of Corrosion
External Microbiologically- influenced Corrosion (MIC)
Microbiologically- influenced Corrosion happens when metal is introduced to water and microorganisms in the water attach to the metal creating a biofilm that can oxidize metals, create acid or hydrogen sulfide, all reactions that will weaken and break down the metal the microorganisms are attached to.
Hydrogen Induced Cracking
Hydrogen corrosion cracking is also known as step-wise cracking and usually happens on steels graded 22 or higher on the Rockwell C scale. Hydrogen atoms at high temperatures can easily diffuse into metals. The hydrogen atoms then combine to form molecules, causing pressure to build from within the metal and cracking it. This creates the hydrogen blisters, and the blisters spread until they are connected to other cracks within the surface, as seen below.
Stress Corrosion Stress corrosion cracking (SCC) is the combination of hydrogen induced cracking and metal that has been strained due to applied stress. For stress corrosion to occur the material has to be vulnerable, in an environment that causes stress corrosion, and there needs to be enough pressure to induce it. Stress corrosion, while not a common occurrence can be very costly as it has the ability to cause large breakdowns and mechanical failures. It can be difficult to detect with a regular inspection as it causes very little metal loss.
Flow Induced Corrosion
Flow-induced corrosion happens because of the consistent flow of a corrosive fluid, like water, against a metal surface. Also known as erosion corrosion, the flow removes the protective film, on the metal’s surface causing an increase in the attack rate of corrosion.
Internal Pitting & Uniform Corrosion
Uniform corrosion is when corrosion spread across a surface evenly, and often times covers the whole or a large part of the surface. It is predictable and easily measured so that preventative measures to keep metals from failing can be taken.
Pitting corrosion happens when holes or cavities are produced in a material. It is localized and more difficult to detect or predict than uniform corrosion. The types of pitting corrosion include:
Narrow and deep
Shallow and wide
Vertical grain attack
Horizontal grain attack
Pitting can be caused by chemical or mechanical damage to the naturally formed oxide film, protective coating, or nonmetallic additions to the uniform structure of the metal. Stress corrosion cracking can also be induced by pitting.
Borescope Inspections for Corrosion
The X1 Articulating Video Borescope is ideal for corrosion inspection. It’s magnetized base allows it to be attached to any metal surface for ease of use and has three different tube lengths, and two different tube diameters for your inspection needs. The X1 series borescope can be used to check for corrosion in engine valves, combustion chambers, engine crankcase, cylinder walls and any other engine part that may be hard to reach without taking apart the engine. With its 3.5 solid state high resolution LCD monitor, quality picture will provide a clear inspection that will allow for the identification of corrosion early on so that preventative measures may be taken to prevent engine and machine failure. Demo the X1 Series Borescope today!
Learn more about articulating borescopes and their real-world applications
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