【Daily Question】12.20 Factors affecting metal corrosion (3 wealth points for replies + additional 3–8 wealth points for correct answers)
Thread Content
This post was last edited by lzy4225925 on 2016-1-2 at 10:58. Question: Briefly answer: What are the main factors that affect metal corrosion? 1 Influence of metal and alloy composition: Inclusions in metals can accelerate their corrosion; impurities such as sulfur, oxygen, and hydrogen can damage the protective layer on the metal surface. The corrosion rate of an alloy is closely related to its composition – for example, in Fe-Cr alloys, an increase in the Cr content significantly enhances the alloy’s corrosion resistance ； 2. Effects of deformation and stress: The high internal stresses resulting from cold and hot working accelerate the corrosion process; in the presence of substances such as hydrogen sulfide, this can even lead to stress corrosion cracking. 3. Effects of medium composition and concentration: Most metal materials have a certain range of compatibility with corrosive media. For example, carbon steel dissolves rapidly in dilute sulfuric acid but remains stable in concentrated sulfuric acid; stainless steel is highly resistant to corrosion in medium- and low-concentration acids, but it is not resistant to corrosion in concentrated acids ； 4. Effect of temperature: An increase in temperature accelerates electrochemical corrosion. 5. Effect of pressure: An increase in pressure leads to a higher gas concentration, which in turn speeds up metal corrosion. 6. Effect of flow rate: An increase in flow rate causes the corrosion products to break off or erode the protective film; extremely high flow rates can also lead to wear corrosion, all of which accelerate the rate of metal corrosion. Requirements and hints for answering the question: Requirement: Answer freely. Hint: Analysis can be conducted from various aspects such as internal and external factors, the manufacturing process, and operating conditions. Note: We will close this thread after 24 hours, during which time we will announce the best response (the most helpful answer), along with the solution and explanations. Other details: Recruitment for moderators & technicians in the [Mechanical Section] – applications and referrals are welcome. 【Valid indefinitely】【If you’re interested, come ahead!】 】(Click to enter) 【Mechanical Zone】Comprehensive guide to preventing equipment freezing over winter in the Mechanical Zone 【Mechanical Zone】Call for collection of essential knowledge 【Seeking manufacturer sponsors】(Ongoing campaign) 【Rewards for recommending valuable content; the “Active Users and Weekly Stars” competition in the Mechanical Zone has begun】
Important factors affecting metal corrosion: impurities in the metal’s composition, gaps and rough surfaces; an operating environment with corrosive liquids or gases, and large temperature fluctuations
The main factors affecting metal corrosion include: ① Metal-specific factors (properties, composition, structure, surface condition, deformation, and stress, etc.)； ②Environmental factors (medium composition, concentration, pH, temperature, pressure, velocity of electrolyte solution movement, etc.) ； ③ Surface condition (surface defects and contaminants on metal materials such as dust, oil, moisture, oxide scale, rust, and burrs). The surface condition of metal materials has a significant impact on the corrosion process; by altering the surface condition of metals, it is possible to influence or even change their corrosion behavior. Metal cleaning is based on this principle.
The essence of metal corrosion is the process in which metal atoms lose electrons and get oxidized. Metal corrosion can generally be divided into chemical corrosion and electrochemical corrosion. Electrochemical corrosion is the most common and important type of metal corrosion. The corrosion that occurs in steel in humid air is a prominent example of electrochemical corrosion. In humid air, a thin layer of water forms on the surface of steel. If this layer of water is highly acidic, H+ ions gain electrons and hydrogen gas is produced; this type of electrochemical corrosion is known as hydrogen evolution corrosion； When this water film is weakly acidic or neutral, it can dissolve more oxygen; in such cases, O2 gains electrons and OH– is released. This type of electrochemical corrosion is known as oxygen absorption corrosion, and it is the main cause of steel corrosion.
Metal corrosion is caused by various internal and external factors, which can be summarized as follows: ① The chemical composition and structure of the metal material itself； ②Metal surface finish (oxygen concentration difference cell corrosion) ； ③Components of the solution in contact with the metal surface and pH value ； ④Ambient temperature and humidity ； ⑤Various environmental media in contact with the metal surface.
Metal corrosion is caused by various internal and external factors, which can be summarized as follows: ① The chemical composition and structure of the metal material itself； ②Metal surface finish (oxygen concentration difference cell corrosion) ； ③Components of the solution in contact with the metal surface and pH value ； ④Ambient temperature and humidity ； ⑤Various environmental media in contact with the metal surface.
Generally, an increase in temperature accelerates the rate of chemical reactions. Therefore, temperature has a significant impact on chemical corrosion. 2. The composition of the solution in contact with the metal surface and its pH value； 3. Various environmental media in contact with the metal surface. 4. The chemical composition and structure of the metal material itself ； The more reactive a metal is, the easier it is for it to lose electrons and become corroded. If the conductive impurities present in the metal are less reactive than the metal itself, a galvanic cell can form, leading to electrochemical corrosion of the metal. 5. Metal surface finish (oxygen concentration difference cell corrosion) ； Metals are prone to corrosion when exposed to humid air, corrosive gases, and electrolyte solutions.
The essence of metal corrosion is the process in which metal atoms lose electrons and get oxidized. Metal corrosion can generally be divided into chemical corrosion and electrochemical corrosion. Electrochemical corrosion is the most common and important type of metal corrosion. The corrosion that occurs in steel in humid air is a prominent example of electrochemical corrosion. In humid air, a thin layer of water forms on the surface of steel. If this layer of water is highly acidic, H+ ions gain electrons and hydrogen gas is produced; this type of electrochemical corrosion is known as hydrogen evolution corrosion； When this water film is weakly acidic or neutral, it can dissolve more oxygen; in such cases, O2 gains electrons and OH– is released. This type of electrochemical corrosion is known as oxygen absorption corrosion, and it is the main cause of steel corrosion.
There are many factors that affect metal corrosion, the main ones being: ① Factors related to the metal itself (properties, composition, structure, surface condition, deformation, and stress, etc.)； ②Environmental factors (medium composition, concentration, pH, temperature, pressure, velocity of electrolyte solution movement, etc.) ； ③ Surface condition (surface defects and contaminants on metal materials such as dust, oil, moisture, oxide scale, rust, and burrs). The surface condition of metal materials has a significant impact on the corrosion process; by altering the surface condition of metals, it is possible to influence or even change their corrosion behavior. Metal cleaning is based on this principle.
The process of corrosion occurs mainly through two mechanisms: chemical reactions and electrochemical reactions. For example, from the perspective of chemical reactions, the oxidation of iron in the atmosphere can be expressed as follows: at room temperature, Fe + 1/2 O2 + H2O → 2Fe(OH)2; further oxidation leads to 2Fe(OH)2 + 1/2 O2 + H2O → 2Fe(OH)3, which is iron oxide or rust. In terms of electrochemical equations, it is written as: Fe + 1/2 O2 + H2O → Fe2+ + 2OH- → 2Fe(OH)3, again representing iron oxide or rust. In reality, as the environment changes, the various factors that affect corrosion become increasingly complex; these factors can alter the degree or type of corrosion, thereby increasing its severity. For example, the existing moisture and dust in the atmosphere are sufficient to affect the degree of corrosion. However, with the increase in industrialization, various gases and compounds such as sulfur dioxide (SiO2) and hydrogen sulfide (H2S) have been added. Coupled with the high salt content in the island terrain of this region, these substances accelerate material corrosion through electrochemical reactions. For another example, when iron is placed in a high-temperature environment, its mode of corrosion differs from that at normal temperatures; instead, it combines directly with oxygen to form oxides such as Fe2O3 (iron oxide), FeO (ferrous oxide), and Fe3O4 (ferric oxide). In addition to the atmosphere, soil, solutions, pH level, and scattered currents are also environmental factors that affect corrosion. Other factors, such as the material composition of the product, heat treatment effects, and processing effects, also influence the outcome of corrosion.
Temperature, pressure, medium, concentration, pH, structure, operating conditions, and so on