Corrosion and Cathodic Protection
Nineteen videos that explain the theory behind CSWIP 3.1u and 3.2u corrosion questions. I did not make them. I picked them, put them in teaching order, and wrote under each one what the exam wants you to take from it.
Video by video, with the exam angle
Each video teaches the concept. The line under it tells you how that concept turns up in the exam and where divers lose the mark.
Why steel corrodes
Exam angle. Refined steel wants to go back to ore. Rust is that process. Know the four parts of a corrosion cell: anode, cathode, electrolyte, and a metal path. Take one away and corrosion stops.
Exam angle. At the anode an iron atom splits into a positive iron ion that leaves into the seawater and electrons that stay in the steel. That loss of metal is the corrosion. Get comfortable with the word ion before the CP questions.
Exam angle. Two things move in a corrosion cell. Electrons move through the metal. Ions move through the seawater. Keep those two separate and every CP question gets easier.
Electricity basics you need for cathodic protection
Exam angle. Background only. Watch it once so voltage, current and electron flow are not new words when they turn up in the ICCP questions.
Exam angle. A CP survey reads a potential, which is a voltage between the steel and a reference electrode. Understand what a voltage is and the reading stops being a magic number.
Exam angle. ICCP pushes a controlled current onto the structure. Current is the amount of charge moving. Voltage is the push, current is the flow. The exam expects you to keep them apart.
Exam angle. This is the polarity trap. Conventional current runs opposite to electron flow. In an ICCP system current crosses the seawater from anode to structure, while electrons travel through the cable on the negative lead. Wire it backwards and you corrode the structure.
Galvanic corrosion
Exam angle. Two different metals, one electrolyte, one connection. The less noble metal becomes the anode and corrodes. The more noble metal is protected. This one principle is the whole reason a sacrificial anode works.
Exam angle. Learn where steel sits in the galvanic series. Steel is more noble than zinc and aluminium, so they protect it. Steel is less noble than copper, brass and stainless, so those make steel corrode. Exam questions test exactly this ranking.
Localised corrosion: pits, crevices and grain boundaries
Exam angle. Small, deep cavities while the surface looks sound. Very little metal lost overall, but one pit can go most of the way through a wall. It starts at an inclusion, a scratch or a coating break, then runs as an oxygen-concentration cell.
Exam angle. Stagnant water in a tight gap runs out of oxygen. The starved metal inside turns anodic and corrodes faster than the open steel outside. It is driven by an oxygen difference, not by two different metals. That distinction is a common exam question.
Exam angle. Know where to look: under bolt heads and washers, behind gaskets, in lap joints, under marine growth and disbonded coating. The exam asks where crevice corrosion occurs as much as why.
Exam angle. Attack along the boundaries between grains, not the grains themselves. The surface looks sound while the metal weakens from inside. Heat is the usual cause of the changed boundary chemistry.
Exam angle. The case to know is weld decay. Welding heat sensitises a narrow band of the heat-affected zone next to the weld toe, and that band corrodes in a line parallel to the weld. That is why the weld toe and HAZ are priority inspection areas.
Corrosion plus stress, movement or flow
Exam angle. Fretting is tiny repeated rubbing under load, for example a riser clamp. Each movement breaks the oxide film so corrosion restarts on fresh metal. Look for a shiny worn band with reddish-brown debris.
Exam angle. Three things together: a susceptible metal, a tensile stress, and a corrosive environment. Remove any one and it does not happen. The exam likes to ask which three.
Exam angle. Cyclic load plus corrosion. Wave loading on a jacket is the offshore case. Cracks start at stress raisers such as weld toes and grow with every load cycle. Corrosion fatigue makes that happen at lower stress than in air.
Exam angle. Erosion corrosion. Fast turbulent flow scours the oxide film off. Worst at bends and reducers, far worse with sand in the fluid, and softer metals suffer most. Watch where the particles hit the pipe wall in this simulation.
Cathodic protection
Exam angle. The whole of cathodic protection in one video. Make the structure the cathode. Corrosion does not stop, it moves onto a separate anode that corrodes instead. Two ways to do it: sacrificial anodes, zinc or aluminium, no power needed, consumed over time. Or ICCP, external DC, non-consumable anodes, adjustable from the surface. Know the advantages and trade-offs of each.
The course is the exam practice
These videos explain the science. The NDTPrep 3.1u Interactive Theory course puts it in the order the syllabus uses, with the diagrams, the plain-English explanations, and a quiz at the end of every module. Then the Q&A and Flashcards drill it until it sticks.
The videos on this page belong to their creators. I have not edited them. My notes are my own reading of the CSWIP syllabus and are not endorsed by the video makers.