Prevention of Corrosion — Revision Notes

⚡ 30-Second Revision

Barrier Protection: — Physical separation (paint, oil, grease, plastic, metal coatings).
Metallic Coatings:

- Galvanization: Iron coated with Zinc. Zn is more reactive ($E^circ_{\text{Zn}^{2+}/\text{Zn}} = -0.76,\text{V}$). Provides barrier + sacrificial protection. Zn corrodes preferentially. - Tinning: Iron coated with Tin. Sn is less reactive ($E^circ_{\text{Sn}^{2+}/\text{Sn}} = -0.14,\text{V}$). Provides barrier protection only. If scratched, Fe corrodes faster. - Electroplating: Coating with less reactive metals (Ni, Cr, Cu) for barrier + aesthetics.

Sacrificial Protection: — Connect more reactive metal (Mg, Zn, Al) to protected metal. Reactive metal acts as anode and corrodes.
Cathodic Protection (Impressed Current): — External DC source forces metal to be cathode.
Anodic Protection: — For passivating metals (Cr, Ni, Ti). Maintains protective oxide film by external anodic current.
Corrosion Inhibitors: — Chemicals reducing corrosion rate.

- Anodic: (Chromates, Nitrites) Form passive film on anode. Dangerous if underdosed (pitting). - Cathodic: (Bicarbonates, Zn salts) Slow cathodic reaction, form precipitate on cathode. - Mixed: Affect both anode & cathode (organic compounds).

Alloying: — Mix metals to enhance resistance (e.g., Stainless Steel: Fe + Cr forms passive $\text{Cr}_2\text{O}_3$ film).

2-Minute Revision

Corrosion prevention is crucial for metal longevity. The core idea is to break the corrosion cell. Barrier protection is the simplest, using paints, oils, or plastic coatings to physically block oxygen and moisture.

Metallic coatings like electroplating (e.g., nickel, chromium) also act as barriers. A key distinction is between galvanization (zinc coating on iron) and tinning (tin coating on iron). Galvanization offers superior sacrificial protection because zinc is more reactive than iron; if scratched, zinc corrodes instead of iron.

Tinning, however, only provides barrier protection, and a scratch accelerates iron's corrosion as tin is less reactive. Sacrificial protection can also be achieved by connecting a more reactive metal (like magnesium or zinc) as a 'sacrificial anode' to the structure to be protected, forcing the structure to become a cathode.

Cathodic protection can also use an impressed current from an external source. Anodic protection is for specific metals that can form a stable, passive oxide film (like stainless steel due to chromium), maintaining this film electrochemically.

Corrosion inhibitors are chemicals added to the environment; anodic inhibitors (e.g., chromates) form passive films but can cause pitting if underdosed, while cathodic inhibitors (e.g., zinc salts) slow down cathodic reactions.

Finally, alloying (e.g., stainless steel) intrinsically enhances corrosion resistance by forming protective passive layers.

5-Minute Revision

Corrosion, the electrochemical degradation of metals, is prevented by various methods, all aiming to disrupt the corrosion cell. Barrier Protection is the most straightforward, involving physical coatings like paints, greases, or plastics to isolate the metal from oxygen and moisture.

For example, painting a bridge prevents rusting. Metallic coatings also serve as barriers. Electroplating deposits a thin layer of a less reactive metal (e.g., nickel, chromium) for protection and aesthetics.

However, the most critical metallic coatings to differentiate are galvanization and tinning.

Galvanization coats iron with zinc. Zinc is more reactive than iron ($E^circ_{\text{Zn}^{2+}/\text{Zn}} = -0.76,\text{V}$ vs. $E^circ_{\text{Fe}^{2+}/\text{Fe}} = -0.44,\text{V}$). This provides dual protection: a physical barrier and, more importantly, sacrificial protection.

If the zinc layer is scratched, zinc corrodes preferentially, acting as an anode, and protects the iron. In contrast, tinning coats iron with tin ($E^circ_{\text{Sn}^{2+}/\text{Sn}} = -0.14,\text{V}$).

Tin is less reactive than iron, so it offers only barrier protection. If the tin layer is scratched, the exposed iron becomes the anode and corrodes rapidly, often accelerated by the tin acting as a cathode.

Sacrificial Protection is a broader category where a more active metal (e.g., magnesium, zinc) is electrically connected to the metal to be protected (e.g., an underground pipeline). The active metal sacrifices itself by corroding, supplying electrons to the protected structure and making it cathodic.

This is a form of Cathodic Protection. Another form of cathodic protection is the Impressed Current Method, where an external DC power source forces the structure to be cathodic, using an inert anode.

Anodic Protection is a specialized method for metals that exhibit passivation, like stainless steel, titanium, or chromium. These metals form a stable, protective oxide film (e.g., $\text{Cr}_2\text{O}_3$ on stainless steel). Anodic protection involves applying an external anodic current to maintain this passive film, preventing corrosion. This method is used in highly corrosive environments for specific metals.

Corrosion Inhibitors are chemicals added to the environment to reduce corrosion. Anodic inhibitors (e.g., chromates, nitrites) form a passive film on anodic sites, but if underdosed, they can cause dangerous localized pitting.

Cathodic inhibitors (e.g., bicarbonates, zinc salts) slow down cathodic reactions, often by precipitating a protective film. Mixed inhibitors affect both. Finally, Alloying involves mixing metals to enhance inherent corrosion resistance, as seen in stainless steel, where chromium forms a self-healing passive oxide layer, making it highly resistant to rust.

*Worked Mini-Example: Protecting a ship's hull.* To protect a ship's steel hull from seawater corrosion, large blocks of zinc or magnesium are bolted to the hull. Since zinc and magnesium are more reactive than iron (steel), they act as sacrificial anodes.

In the presence of seawater (electrolyte), the zinc/magnesium corrodes: $\text{Zn} \rightarrow \text{Zn}^{2+} + 2e^-$. These electrons flow to the steel hull, making it cathodic and preventing the iron from oxidizing.

The zinc/magnesium blocks are replaced periodically as they are consumed.

Prelims Revision Notes

Prevention of Corrosion: NEET Revision Notes

1. Barrier Protection:

* Principle: Physical separation of metal from corrosive environment (O$_2$, H$_2$O). * Methods: * Painting/Varnishing: Common, cost-effective. Primer often contains inhibitors (e.g., red lead).

* Oiling/Greasing: For machinery, tools. Forms hydrophobic layer. * Plastic/Rubber Coating: Durable, chemical resistant (e.g., PVC, epoxy). * Metallic Coatings: * Electroplating: Depositing a thin layer of a less reactive metal (Ni, Cr, Cu, Ag, Au) electrolytically.

Primarily barrier + aesthetics. * Tinning: Coating iron with tin (Sn). Sn is *less reactive* than Fe ($E^circ_{\text{Sn}^{2+}/\text{Sn}} = -0.14,\text{V}$ vs. $E^circ_{\text{Fe}^{2+}/\text{Fe}} = -0.

44, ext{V}$). Provides **barrier protection ONLY**. If scratched, Fe corrodes *faster* (Sn acts as cathode). * **Galvanization:** Coating iron with zinc (Zn). Zn is *more reactive* than Fe ($E^circ_{\text{Zn}^{2+}/\text{Zn}} = -0.

76, ext{V}$). Provides barrier + sacrificial protection. If scratched, Zn corrodes preferentially (Zn acts as anode), protecting Fe.

2. Sacrificial Protection (Cathodic Protection - Sacrificial Anode Method):

* Principle: Connect the metal to be protected to a *more electrochemically active* metal (sacrificial anode). The active metal corrodes, supplying electrons to the protected metal, making it cathodic. * Sacrificial Anodes: Mg ($E^circ = -2.37,\text{V}$), Zn ($E^circ = -0.76,\text{V}$), Al ($E^circ = -1.66,\text{V}$) for protecting iron. * Applications: Underground pipelines, ship hulls, water tanks.

3. Cathodic Protection (Impressed Current Method):

* Principle: External DC power source forces the metal structure to be protected to act as a cathode. An inert anode (e.g., graphite) is used. * Applications: Large pipelines, storage tanks, reinforced concrete.

4. Anodic Protection (Passivation):

* Principle: Applicable to metals that exhibit passivation (form a stable, protective oxide film, e.g., $\text{Cr}_2\text{O}_3$ on stainless steel). An external anodic current maintains the passive state. * Metals: Stainless steel, titanium, chromium, nickel. * Caution: Requires careful potential control; too high potential can cause transpassivation (film breakdown).

5. Corrosion Inhibitors:

* Principle: Chemicals added to corrosive environment to reduce corrosion rate. * Types: * Anodic Inhibitors: (e.g., Chromates, Nitrites, Phosphates, Molybdates). Promote passivation at anodic sites.

Danger: If underdosed, can cause severe localized corrosion (pitting) at unprotected anodic sites. * Cathodic Inhibitors: (e.g., Bicarbonates, Zinc salts, Arsenic compounds). Slow down cathodic reaction (e.

g., O$_2$ reduction, H$_2$ evolution) or precipitate protective film on cathodic sites. Generally safer if underdosed. * Mixed Inhibitors: (Many organic compounds like amines, thiols). Adsorb on surface, affecting both anodic and cathodic reactions.

6. Alloying:

* Principle: Mixing metals to enhance inherent corrosion resistance. * Examples: * Stainless Steel: Iron + Chromium (>10.5%) + Nickel. Chromium forms a stable, self-healing passive $\text{Cr}_2\text{O}_3$ layer. * Duralumin: Al alloy for strength and corrosion resistance.

Key Concept: Relative reactivity of metals (electrochemical series) is crucial for understanding sacrificial protection and galvanic corrosion. More negative $E^circ$ = more reactive = greater tendency to act as anode.

Vyyuha Quick Recall

To PREVENT CORROSION, remember P-A-S-S-I-O-N:

Paint & Plating (Barrier Protection) Alloy (Stainless Steel) Sacrificial Anode (Galvanization, Mg/Zn blocks) Surface Treatments (Oiling, Greasing) Inhibitors (Anodic, Cathodic, Mixed) Outside Current (Cathodic Protection - Impressed Current) Noble Potential (Anodic Protection/Passivation)