OXIDACIÓN Y CORROSIÓN 5 min

Corrosion and Oxidation of Metals

Understanding Corrosion Mechanisms

• The process of corrosion is initiated in an atmosphere containing oxygen and water vapor, which leads to the formation of rust. Condensation occurs when water vapor meets a cold surface.

• Oxygen can also be condensed into liquid form by cooling air below -183 degrees Celsius.

• Magnesium reacts with oxygen from the air when heated, resulting in magnesium oxide formation.

• Heating carbon steel will similarly lead to oxidation, producing iron oxide.

• When red-hot steel interacts with air's oxygen inside a furnace, it creates a temporary glow that diminishes upon contact with cold water, leading to rapid oxidation.

Characteristics of Different Steels

• Carbon steel consists of approximately 95% iron; thus, its oxidation results in iron oxide formation.

• The weight of carbon steel increases during oxidation due to added oxygen. Initially, oxidation occurs rapidly but slows as the layer thickens.

• The initial ease of reaction between iron and oxygen decreases as iron oxide builds up, yet oxidation continues until complete destruction may occur.

• Stainless steel oxidizes at a significantly slower rate compared to carbon steel.

• Chromium in stainless steel forms an oxide layer that inhibits further reactions with oxygen and enhances heat resistance. Carbon steel should not be used in high-temperature environments like furnaces; hence stainless steel is preferred for furnace floors.

Experimental Demonstration of Corrosion

• An experiment illustrates the difference between carbon and stainless steels using three tubes each containing samples of both types.

• Air is introduced into cold water through agitation. One tube contains carbon steel while another contains stainless steel; boiling removes dissolved air from the water before sealing them tightly without any air present.

• Only the tube with carbon steel oxidizes due to wet corrosion being an electrochemical process requiring both water and oxygen.

Electrochemical Processes Involved

• When submerged in aerated water, atoms from carbon steel dissolve into positively charged ions (cations), leaving behind negatively charged electrons that migrate through the metal seeking reaction sites with oxygen and water to form hydroxyl ions.

• Iron oxide does not protect against further corrosion; thus, degradation continues unabated once formed.

Factors Affecting Oxidation Rates

• Boiled water lacks dissolved oxygen necessary for hydroxyl ion formation; therefore no iron oxide forms on submerged carbon steel preventing its oxidation process entirely.

• Stainless steels resist corrosion because chromium atoms oxidize forming a protective passive layer that prevents further ionization and stops corrosion effectively known as "passive layer."

Galvanic Corrosion Insights

• If exposed to moderate chromium oxides, this passive layer regenerates continuously providing ongoing protection against corrosion making stainless steels suitable for abrasive industries or daily applications.

• Metals can be classified based on their voltages measured during these processes which help predict outcomes when different metals are placed together within an electrolyte creating galvanic pairs.

Practical Applications & Prevention Strategies

• In galvanic pairs where one metal has higher voltage than another (e.g., aluminum vs stainless), it acts as anode corroding first while electrons flow towards cathode preventing its own deterioration.

• Using aluminum rivets on stainless will result in aluminum corroding faster due to this principle highlighting why such combinations should be avoided unless properly managed.

• To prevent galvanic corrosion keep metals dry or electrically isolated from each other while using stainless rivets with aluminum is acceptable if surface area ratios allow for sufficient thickness loss tolerance.

• Galvanization protects carbon steels by applying zinc coating acting as sacrificial anode corroding preferentially over underlying metal until depletion occurs rendering protection ineffective eventually.

Corrosion Protection in Stainless Steel

Importance of Corrosion Protection

• The stainless steel tube is protected from galvanic corrosion caused by titanium and other noble metals.

• There are instances where galvanic protection cannot be utilized effectively.

Special Properties of Stainless Steel

• In situations lacking galvanic protection, a special oxide layer forms on stainless steel, enhancing its resistance to corrosion.

• Various types of stainless steel have been developed to withstand different corrosive environments and working conditions.

Benefits of Using Stainless Steel