Back in 2017, I wrote an article about finding severe corro­sion in my Twin. By severe, I mean that I came very close to catastrophic flight control failure as a direct result of that corrosion. I had owned my Twin eight or nine years at the time, as I recall, and it was missed on that many annual inspec­tions. It was about that time that I moved to a new airport and new shop. My new mechanic found it on the first annual in­spection. My new shop repaired it for about $8,000 or so. The quality of the repair was perfect, and I was pleased.

Since then, I’m always looking for corrosion, and with the Twin gone and a 182 in my hangar, I’m still looking for it. Re­cently, I found a small amount of corrosion and I’m going to take care of it.

Since I haven’t dealt with corrosion since 2017, I was, if you’ll pardon the expression, a bit rusty on the topic. I went to get the bible on corrosion from the FAA website, and all owners should read it. You can download it free from FAA.gov. Search for Advisory Circular 43-4A, titled “Corrosion Control for Aircraft.” Written in 1991, this is the seminal document on the subject.

If you’re new to this subject, this document is so scary that you’ll want to run out to your plane and drape your body over the wings. We pilots are tough, though, so spend an hour and read this document. I’m going to cover one section in the doc­ument entitled, “Forms of Corrosion.” I won’t try to teach you about corrosion because I could never do as good a job as this circular. However, I’ll give you a taste of what it includes and tell you a little about each form of corrosion that attacks our flying machines.

Forms of Corrosion

A. Uniform Etch Corrosion. Uniform etch corrosion results from a direct chemical attack on a metal surface and in­volves only the metal surface.

B. Pitting Corrosion. The most common effect of corrosion on aluminum and magnesium alloys is called pitting.

C. Galvanic Corrosion. Galvanic corrosion occurs when two dissimilar metals make electrical contact in the presence of an electrolyte.

D. Concentration Cell Corrosion. Concentration cell corro­sion is corrosion of metals in a metal-to-metal joint, cor­rosion at the edge of a joint even though joined metals are identical, or corrosion of a spot on the metal surface cov­ered by a foreign material.

E. Intergranular Corrosion. Intergranular corrosion is an at­tack along the grain boundaries of a material.

F. Exfoliation Corrosion. Exfoliation corrosion is an ad­vanced form of intergranular corrosion in which the sur­face grains of a metal are lifted by the force of expanding corrosion products occurring at the grain boundaries just below the surface.

G. Filiform Corrosion. Filiform corrosion is a special form of oxygen concentration cell corrosion or crevice corrosion that occurs on metal surfaces having an organic coating system.

If this isn’t enough to keep you up at night worrying about your airplane, take heart that you can get control of this relatively eas­ily. First, read the circular, and study the photos and diagrams. If you have questions, log on to our online forum and ask them. We area, and some even hold PhDs in related subjects. Our associa­tion A&P/IA, Erich Rempert, is also an expert in this area.

So where did I find corrosion on my plane? I found galvanic corrosion on my ailerons. Galvanic corrosion occurs when two dissimilar metals make electrical contact in the presence of an electrolyte. If you look at the exterior of your airplane, you’ll see perhaps as many as hundreds of screws. Check where these steel screws meet the aluminum skin around them. If the paint is stained brown, then you have corrosion. Not to sound overly dramatic, but if the screws have never been replaced, and your plane is 40 or 50 years old, you could wind up with unrepairable corrosion all over the aircraft.