Corrosion is a circuit

Forty-six elements, tied largest in General, and eight of them are Risk Management — every one about chemicals.

The subject looks like a list of products and procedures. One idea makes it predictable instead.

1. Anode, cathode, electrolyte, path

Corrosion is an electrochemical reaction, and it needs four things at once:

Remove any one and corrosion stops. That single fact organizes the entire subject:

• Coatings, primers and sealants remove the electrolyte's access.
• Conversion coatings change the surface so it is less willing to be an anode.
• Barriers between dissimilar metals break the path (§3).
• Drainage and ventilation remove the electrolyte itself — which is why blocked drain holes are a corrosion finding and why K4's prone areas are prone.

And it lets you predict where corrosion will be on an aircraft you have never seen. Anywhere water collects and does not drain. Anywhere two different metals meet. Anywhere a coating is broken. Anywhere warm, humid air condenses — the underside of a wing skin, the bottom of a fuselage, battery compartments, lavatories, bilges.

A candidate who holds the model predicts prone areas. One who memorized K4's list finds only what the list mentions.

2. This unit is two subjects, and they join at surface preparation

Corrosion — K2–K9, K12, K13, and the inspection and treatment skills — is about electrochemistry, detection and arrest.

Finishing — K14–K24 and the preparation skills — is about coating systems, adhesion and appearance.

They share chemistry, ventilation and PPE and little else. The join is K17 and K19: surface preparation, where a corrosion decision determines a finishing outcome, and where a finishing shortcut produces a corrosion problem three years later.

K24, control surface balance after refinishing, sits in neither half comfortably. It belongs with AF-C §4, and it is the element where paint becomes a weight and balance problem and then a flutter problem.

3. Dissimilar metals, and what a barrier actually does

K12. When two different metals are in electrical contact with an electrolyte present, one of them becomes the anode and corrodes preferentially. Which one is set by their relative position in the galvanic series, and the further apart they are, the faster it goes.

This is why an aluminum structure with steel fasteners, or a magnesium component against aluminum, is a designed-in corrosion risk that has to be managed rather than avoided.

A barrier works by breaking the conductive path — a sealant, a primer, a cadmium plating, a non-metallic washer. It does not neutralize anything; it just stops the circuit completing.

So a barrier that is broken anywhere has failed everywhere along that joint. A scratch through primer at one fastener puts the whole faying surface back into the circuit, which is why AF-A's repairs are sealed and primed rather than merely fitted.

4. R3 and K11 — the cleaner that causes the damage

Identification of materials and processes to be used for cleaning or corrosion treatment, and improper use of cleaners on aluminum or composite materials.

Strong alkaline cleaners attack aluminum. Some solvents attack composite matrix, and some attack transparent plastics — crazing an acrylic windscreen is permanent and expensive (AF-B).

So the material must be identified before the cleaner is chosen, which makes GEN-E Concepts §5's material identification a prerequisite for touching anything in this unit.

The general rule: use the product the manufacturer approves for that material and that job. "It cleaned it" is not evidence that it was suitable; the damage from the wrong cleaner is usually invisible at the time.

5. Why surface preparation decides how long a finish lasts

K17 and K19, and the reason candidates skip them.

Adhesion is mechanical and chemical, and both are established before any coating goes on. A finish applied over grease, oxide, moisture or dust bonds to the contaminant rather than the metal, and the contaminant is not attached to anything.

The failure appears months later as lifting, peeling or blistering — and that is precisely the problem. The feedback loop is too slow to learn from experience. A shortcut taken today looks identical to a job done properly, and the difference emerges long after anyone connects the two.

That is why it has to be taught explicitly rather than absorbed, and why K18's ambient conditions matter: applying a coating outside its temperature and humidity window produces a finish that looks right and is not.

6. Eight risk elements, and the one a school forgets

R1 health, R2 ventilation, R3 material identification, R4 SDS-directed PPE, R5 flammable chemicals, R7 PPE with paints and solvents, R8 application of finishing materials — all straightforward, all serious, all governed by the safety data sheet.

R6 is the one that gets forgotten: disposal of chemicals and waste materials.

It is a regulatory obligation, and it costs money per event. Solvent, spent penetrant, used cleaner, contaminated rags and stripper are all controlled waste. In a shop, including the one where you do your practical work, the shop disposes of its own waste under its lease and local rules, and "it went in the bin" is never the answer. Learn where each waste goes before you make any.

An earlier design of this program, our mobile delivery plan (superseded 2026-09-18), would have carried its waste to host sites, and its costing, our unit economics (superseded 2026-09-18), had no line for it. PP-G §2 adds used oil to the same obligation.

7. Detection before treatment

K6, S1 and S5. Treating visible corrosion without establishing its extent leaves the rest behind, and it leaves it behind under a fresh coating where nobody will look again.

Corrosion spreads under coatings and along faying surfaces from a point that may be much smaller than the damage. So the sequence is: find it, establish how far it goes, remove all of it, verify removal, treat, and restore the protective system — and only then refinish.

GEN-K Concepts §2's inspection technique applies directly, and the specimen library (the equipment and materials specification §B2) is what makes extent assessable, because a candidate must decide where the corrosion stops rather than confirm that it is present.

8. No element requires a paint booth

Worth stating in the curriculum as well as in the equipment description, because the natural assumption runs the other way.

Eleven knowledge elements cover finishing. It reads like a spray-booth subject. But every station skill in this unit is preparation or a wipe-and-brush application — masking, etch and conversion coating, corrosion-preventive compound, surface preparation.

Applying a topcoat appears only as knowledge (K23), never as a skill. A ventilated bench, spill containment and the PPE the risk elements demand are the whole facility requirement (the equipment and materials specification).

9. What consumes this unit

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