Korrosionsschutz: aktiv und passiv

Introduction to Iron and Steel

Overview of Iron and Steel Production

• The discussion begins with an introduction to iron, various types of steel, corrosion protection methods (active and passive), and the anodizing process.

• Iron ore is processed in a blast furnace to produce pig iron. Cast iron, which contains 2-4% carbon, is brittle and can be cast but not forged.

Characteristics of Steel

• Steel has a lower carbon content (0.2-2%), making it malleable and ductile. There are approximately 2000 different types of steel based on alloying elements.

• Alloying elements like chromium (for corrosion resistance), nickel (for ductility), and manganese (to increase hardness) significantly affect the properties of steel.

Stainless Steel and Its Properties

Understanding Stainless Steel

• "Edelstahl" or stainless steel refers to high-quality steel that is resistant to corrosion due to its chromium content.

• V2A steel, a type of chromium-nickel steel, forms a thin protective layer of chromium oxide that prevents rust.

Corrosion Protection Methods

Coating with Noble Metals

• One method for reducing corrosion involves coating iron with a more noble metal such as tin (Sn).

• If the coating remains intact, it protects the underlying metal; however, if damaged, localized corrosion can occur as electrons flow from the less noble iron to the more noble tin.

Application Example: Tin Coating

• An example includes tin-coated sheet metal used in food packaging; while effective at protecting against rust, it can dissolve in acidic solutions without additional coatings like plastic lacquer.

Using Less Noble Metals for Corrosion Protection

Zinc Coating Method

• Another approach uses zinc as a coating material since it is less noble than iron. Zinc acts as an anode during oxidation.

• In this case, zinc oxidizes instead of iron when exposed to corrosive environments; however, zinc-coated materials are unsuitable for direct contact with food due to rapid dissolution in acidic conditions.

Magnesium as an Anode for Corrosion Prevention

Magnesium's Role in Corrosion Protection

• Magnesium serves as another sacrificial anode due to its high reactivity. It dissolves slowly while protecting other metals by reacting with oxygen.

• This process helps reduce any formed iron ions back into elemental iron while magnesium itself oxidizes.

Corrosion Protection Techniques

Sacrificial Anodes and Corrosion Prevention

• The effectiveness of sacrificial anodes, such as magnesium, in preventing rusting until they are consumed is discussed. Alternative methods include using external current for corrosion protection.

• Zinc and aluminum create a protective passive layer through oxidation under normal environmental conditions, which will be explored further.

Galvanization Process

• The process of galvanizing involves the electrolytic deposition of less noble metals like zinc onto steel, creating a barrier that separates iron from its environment.

• Zinc corrodes in air and is often passivated with a thin chromium-based layer to protect against oxidation.

Limitations of Aluminum Coating

• While aluminum forms a dense protective oxide layer (self-passivation), it is less effective than zinc as a sacrificial anode for iron due to rapid self-passivation.

• Damage to the aluminum coating can lead to rusting of the underlying iron, contrasting with zinc's role as an active sacrificial anode.

Experimental Observations on Sacrificial Anodes

• An experiment illustrates how unedged metals like zinc serve as sacrificial anodes connected to iron components in applications such as pipelines or bridges.

• A local element formed between iron nails and copper accelerates corrosion, while nails connected to zinc show no discoloration until the zinc is fully dissolved.

Passive Corrosion Protection Methods

• Passive protection involves applying outer protective layers (e.g., paints or coatings) that isolate metal from moisture and corrosive elements.

• Various methods including galvanization and hot-dip galvanizing create thicker protective layers compared to electroplating.

Advanced Coating Techniques

• Glass coatings involve melting silicates onto metals for added protection; phosphoric acid can also form insoluble phosphate layers that shield against corrosion.

• The reaction between ferric oxide (rust) and phosphoric acid produces ferric phosphate, acting as a rust converter.

Electrolytic Oxidation of Aluminum (Eloxal)

• Eloxal refers to the electrolytic oxidation process that creates a hard, corrosion-resistant oxide layer on aluminum through anodization.

• This process results in a porous structure that can be dyed for aesthetic purposes while enhancing durability compared to standard oxidation processes.

This structured summary captures key concepts related to corrosion prevention techniques discussed in the transcript while providing timestamps for easy reference.

Chemical Processes in Aluminum Oxidation and Reduction

Overview of Chemical Reactions

• The aluminum anode acts as the positive pole in the chemical process, where oxidation occurs. This results in the formation of aluminum oxide (Al2O3).

• In this reaction, aluminum ions are +3 and oxygen ions are -2, combining to form aluminum oxide with a stoichiometric requirement of six electrons.

• The reaction involves two aluminum atoms and three water molecules to produce aluminum oxide and hydrogen gas.

Reaction Dynamics

• The process also includes the reduction at the cathode (negative pole), where six protons (H+) react with six electrons to generate three molecules of hydrogen gas (H2).

• Overall, the complete reaction can be summarized as: Aluminum reacts with water to yield aluminum oxide and hydrogen gas.