Corrosion Protection

Introduction

Corrosion needs four things: an anode, a cathode, an electrolyte and an electronic path. Remove or weaken any one and the corrosion cell stops. Every protection method, from a coat of paint to the blocks of zinc bolted to a ship's hull, can be understood as an attack on one part of this cell. With the thermodynamics and kinetics of the previous pages, you can now see exactly how each strategy works and when it fails.

Core explanation

1. Barrier coatings. Paints, polymer linings, enamel and grease exclude water and oxygen and raise the resistance of the electrolyte path. They work only while intact. At a scratch, the small exposed area of steel can become an anode for a larger cathode elsewhere, so damaged coatings may produce concentrated local attack. Modern paint systems therefore combine a primer containing inhibitors with barrier top coats.

2. Metallic coatings. A coating of a more noble metal, such as tin on steel food cans, is an excellent barrier but becomes dangerous when scratched: the exposed steel is anodic to the tin and corrodes faster. A coating of a less noble metal, such as zinc in galvanised steel, protects in two ways: as a barrier, and as a sacrificial anode that continues to protect exposed steel at scratches. Zinc also corrodes slowly itself, because it forms a basic zinc carbonate film in air.

3. Passivation. Some metals form an extremely thin (a few nanometres) oxide film that is adherent, non-porous and poorly conducting to ions. On an Evans diagram, once the potential rises past a critical value the anodic current drops by several orders of magnitude. Stainless steel contains at least about 10.5% chromium, which forms a Cr₂O₃-rich passive film; if scratched, it re-forms within moments in the presence of oxygen. Aluminium and titanium behave similarly. Passivity can fail locally, however: chloride ions can break the film at weak points and cause pitting, which is why grades containing molybdenum are chosen for marine use.

4. Cathodic protection. Lowering the potential of a structure towards the immunity region of its Pourbaix diagram suppresses the anodic reaction. Two ways exist:

- Sacrificial anodes of zinc, aluminium or magnesium alloys are electrically connected to the steel. They form a galvanic couple with a mixed potential more negative than steel's corrosion potential, so the anode corrodes and the steel becomes a cathode. The anodes are consumed and must be replaced. - Impressed current uses a DC supply and long-lasting inert anodes (such as mixed-metal oxide coated titanium) to drive electrons into the structure. It suits large structures such as pipelines and can be adjusted using reference-electrode measurements.

Over-protection must be avoided: pushing the potential too negative evolves hydrogen, which can blister coatings and embrittle high-strength steels.

5. Modifying the environment. Removing dissolved oxygen from boiler water, raising the pH, drying the air or adding inhibitors all reduce the cathodic or anodic rate. Anodic inhibitors promote passivation, while cathodic inhibitors hinder oxygen reduction or precipitate a film at cathodic sites.

6. Design. Avoiding crevices, water traps and unfavourable dissimilar-metal contacts, and insulating dissimilar metals from each other, prevents many problems before they begin.

Step-by-step reasoning

To choose a protection method:

1. Identify the environment: atmospheric, buried, immersed, acidic or chloride-rich. 2. Decide which element of the corrosion cell is easiest to remove. 3. For buried or immersed steel, consider coatings combined with cathodic protection. 4. For atmospheric exposure, consider galvanising, paint or a passivating alloy. 5. Check for side effects such as galvanic coupling or hydrogen embrittlement.

Visual explanation

Picture a scratched steel sheet in two versions. On tin-plated steel, arrows of current flow from the exposed steel into the tin, and a pit deepens at the scratch. On galvanised steel, arrows flow from the zinc into the steel, and the zinc around the scratch slowly thins while the steel stays bright.

Real-world analogy

A sacrificial anode is like a bodyguard who takes the blows intended for someone else. As long as the bodyguard is present and in contact, the protected person is safe; when the bodyguard is worn out, he must be replaced.

Real-world example

Offshore oil platforms and ship hulls carry large aluminium or zinc alloy blocks welded to the steel below the waterline. Long buried pipelines typically use a coating plus impressed-current cathodic protection, with engineers routinely checking the pipe-to-soil potential against a copper/copper sulfate reference electrode.

Why?

Why does tin plating accelerate corrosion once scratched, while zinc does not? Tin is more noble than iron, so at a scratch iron becomes the anode of the couple. Zinc is less noble than iron, so at a scratch zinc is the anode and iron is the protected cathode.

Common misconception

"Stainless steel cannot rust." Stainless steel relies on a passive film. In chloride-rich, oxygen-poor conditions, such as crevices in seawater, the film can break down and pitting or crevice corrosion occurs.

Worked example

Question: A zinc anode must supply a protection current of 0.50 A for one year. What mass of zinc is consumed? (Zn = 65.4 g/mol, n = 2; one year ≈ 3.15 × 10⁷ s)

Reasoning: Charge = 0.50 × 3.15 × 10⁷ ≈ 1.58 × 10⁷ C. Moles of electrons = 1.58 × 10⁷ ÷ 96 485 ≈ 163 mol. Moles of Zn = 81.6 mol. Mass = 81.6 × 65.4 ≈ 5340 g.

Answer: About 5.3 kg of zinc, before allowing for the anode's efficiency of less than 100%.

Quick check

1. Why does connecting magnesium blocks to a buried steel tank slow the corrosion of the tank? Answer: Magnesium is less noble, so it becomes the anode and corrodes while the steel is held cathodic.

Exam focus

Link each protection method to the part of the corrosion cell it interrupts. Explain why galvanising protects at scratches but tin plating does not, and compare sacrificial and impressed-current cathodic protection, including when each is appropriate.

Advanced insight

The passive film on stainless steel is a semiconductor only a few nanometres thick, and its stability depends on alloy composition. The pitting resistance equivalent number, a weighted sum of chromium, molybdenum and nitrogen contents, is used to rank grades for chloride environments, a practical expression of how composition controls film chemistry.

Summary

Corrosion protection works by removing part of the corrosion cell. Barrier coatings exclude the electrolyte, passivating films slow the anodic reaction, sacrificial anodes and impressed currents make the structure a cathode, and inhibitors or environmental control slow either half-reaction. Each method has limits, including scratches, chloride attack, anode consumption and over-protection.

Practice questions

1. State two ways in which galvanising protects steel. Answer: Zinc acts as a barrier to water and oxygen, and it acts as a sacrificial anode at scratches. 2. Why is chromium essential in stainless steel? Answer: It forms a thin, adherent chromium oxide passive film that re-forms when damaged and greatly slows corrosion. 3. Give one advantage of impressed-current protection over sacrificial anodes for a long pipeline. Answer: The protective current can be adjusted and the inert anodes last much longer, so it can protect large structures in high-resistance soils. 4. Explain why cathodic protection should not push the potential too negative. Answer: Excessive cathodic polarisation evolves hydrogen, which can damage coatings and cause hydrogen embrittlement of the steel.