Coating Fundamentals and Corrosion Basics
What the exam expects
You must be able to explain why a coating protects steel, not just name products. The core mechanism is barrier exclusion plus, for many systems, electrochemical or inhibitive action. Understand the corrosion cell: an anode (where metal dissolves), a cathode (where reduction occurs), an electrolyte path, and an electrical connection through the metal. Remove any one element and corrosion stops - this logic drives nearly every question about surface preparation, coating selection, and defect consequences.
Know the common corrosion forms: uniform, galvanic (dissimilar metals coupled in an electrolyte), crevice, pitting, and underfilm (filiform or blister-driven) corrosion. Galvanic questions often hinge on the anode/cathode area ratio - a small anode coupled to a large cathode corrodes aggressively.
Coating fundamentals
Distinguish barrier coatings, inhibitive primers (which release passivating species), and sacrificial/zinc-rich coatings that protect steel galvanically. Understand adhesion as mechanical keying plus chemical bonding, and why wet adhesion and osmotic blistering matter in immersion service. Be comfortable with terms like PVC (pigment volume concentration), CPVC (critical PVC), and how exceeding CPVC creates a porous film - a classic trap where a question describes a chalky, permeable coating at high pigment loading.
Traps and a rule of thumb
Traps: confusing the anode with the cathode (metal is lost at the anode); assuming thicker is always better (over-CPVC or excessive DFT can cause cracking or poor adhesion); and mixing up galvanic with stray-current corrosion.
Rule of thumb: in a galvanic couple, the more active metal (lower in the galvanic series in that electrolyte) becomes the anode and sacrifices itself - which is exactly why zinc protects steel.