Corrosion 1 Part 1.WMV

Causes and Mechanisms of Corrosion 00:11

"Corrosion is a natural phenomenon that attacks metals and alloys in an effort to return them to their original state."

• Corrosion can be understood as an electrochemical process, as it involves the generation of a small amount of electrical current during the corrosion of metals.

• Four essential conditions must be present for corrosion to develop: a readily corroding metal (the anode), another metal with less tendency to corrode (the cathode), an electrical path between the two metals through contact, and a conductive liquid known as the electrolyte.

• A typical example of a corrosion cell on an aircraft could consist of the aluminum skin acting as the anode, a steel fastener as the cathode, with rainwater serving as the electrolyte, establishing an electrical path through metal contact.

• If any of the four conditions—anode, cathode, electrical path, or electrolyte—are eliminated, corrosion can be halted or prevented.

Galvanic Series and Metal Reactions 01:44

"The galvanic series chart shows how different metals compare and how likely they are to corrode."

• Understanding the galvanic series is crucial for recognizing how different metals interact and the potential for corrosion on an aircraft.

• Metals positioned further apart on the galvanic series are more likely to cause one of them to corrode rapidly when joined together. For example, aluminum, being higher on the chart than stainless steel, will act as the anode and corrode quickly in contact with stainless steel in a corrosive environment.

• Conversely, when two metals are closer together on the galvanic series, corrosion will occur but at a slower rate.

Factors Influencing Corrosion Rates 03:00

"Corrosion increases with the time the metal is exposed to a corrosive environment."

• Several factors can influence the rate of corrosion, including the relative sizes of the anodic and cathodic areas. When the cathode is small compared to the anode, the rate of corrosion will be slower; if the opposite is true, corrosion accelerates.

• Temperature also plays a significant role as higher temperatures generally decrease electrical resistance in the electrolyte, leading to increased corrosion rates, especially in tropical climates compared to colder areas.

• Additionally, the duration of exposure to corrosive environments significantly affects corrosion, with longer exposure leading to more severe damage.

Role of Electrolytes in Corrosion 03:49

"Electrolytes are one of the four basic factors that cause metals to corrode."

• Electrolytes can derive from various sources of moisture, including condensation, humidity, rain, or even liquids like coffee.

• Water is the most prevalent electrolyte, but its corrosiveness varies based on chemical composition; for instance, rainwater is more corrosive than tap water due to its lower pH. Saltwater is even more damaging due to its higher salt content, making it a stronger electrolyte.

• Acids, especially strong ones like battery acid, pose severe risks to aircraft metals, while alkaline substances, though generally less corrosive than acids, still attack aluminum and magnesium alloys.

Common Types of Corrosion 05:15

"The most common types of corrosion affecting aircraft metals include uniform, pitting, galvanic, and concentration cell corrosion."

• Uniform corrosion occurs evenly across a metal's surface and is commonly seen as rust on steel. If unaddressed, it can lead to significant degradation of the metal.

• Pitting corrosion is prevalent in aluminum and magnesium alloys, appearing as small dots initially but can escalate to a larger deteriorative effect over time.

• Galvanic corrosion arises when two different metals are in contact with an electrolyte, leading to accelerated corrosion of one metal.

• Concentration cell corrosion happens in crevices where moisture accumulates, creating localized corrosion cells.

• Advanced forms of corrosion include intergranular corrosion, reminiscent of cancer as it follows the grain boundaries of the metal, and exfoliation, where surface grains expand due to corrosive products.

• Stress corrosion cracking and corrosion fatigue occur under specific mechanical stress conditions, exacerbating regular corrosion processes and leading to material failure.