First of all, it must be clear that stainless steel is not completely stainless. In our general sense, stainless steel refers to steel that can resist the corrosion of the atmosphere and weak corrosive media. Stainless steel with corrosion rate less than 0.0 1 mm/year is called "complete corrosion resistance", and stainless steel with corrosion rate less than 0. 1 mm/year is called "corrosion resistance". Therefore, stainless steel cannot be corroded, but the corrosion rate is slow.
Go back to the three questions you doubt:
1) chromium cannot completely cover the matrix. Your point of view should be correct, but as we said above, we just need to make it corrode slowly enough. Therefore, stainless steel has requirements for chromium content, that is, the chromium content must reach a certain amount, from quantitative to qualitative.
2) The concept of corrosion classification is involved here. Corrosion can be divided into two categories according to its chemical principle: chemical corrosion and electrochemical corrosion. Chemical corrosion is a process in which metals react with media and are destroyed, such as high-temperature oxidation decarbonization of steel and corrosion in oil and gas. Typical chemical reactions are: 4Fe+3O2═2Fe2O3.
This reactive corrosion does not produce corrosion current and forms a layer of chemical products on the reaction surface. A dense oxide film (passivation film) can prevent further corrosion. Oxides such as SiO2 _ 2, Al _ 2O _ 3 and Cr _ 2O _ 3 have dense structures, larger specific volume than the matrix, and can cover the surface of parts with high chemical stability, thus effectively protecting metal parts from further corrosion. This is the principle of the main points you listed.
Electrochemical corrosion is a process in which the electrochemical process between metal and medium destroys metal, such as atmospheric corrosion and corrosion in various electrolytes. The corrosion encountered in production practice is mainly electrochemical corrosion. In metallic materials, it is produced by the potential difference between electrodes of different metallic elements or phases in metallic materials. This galvanic corrosion is produced between different phases of microstructure, so it is called micro-battery corrosion. Electrochemical corrosion is characterized by the existence of liquid dielectric, the potential difference between different metals or phases and the generation of corrosion current. Because electrochemical corrosion is a more important and common form of metal corrosion, it is extremely important to study the speed of electrochemical corrosion.
The corrosion rate should depend on the amount of metal ions dissolved from the anode per unit time, which is equal to the amount of current passing through the wire per unit time. According to ohm's law, the amount of corrosion should be proportional to the potential difference between cathodes, that is, the electromotive force of primary battery. For the electrochemical corrosion of metal materials, because the cathode and anode of the micro battery are in direct contact, the corrosion current should be very large, that is, the corrosion speed should be very fast. Actually, it's not that fast. This is because the potential of the cathode and anode will change after corrosion, that is, the potential difference will decrease, thus reducing the electromotive force of the primary battery. This change in electrode potential is called polarization. The positive change of anode potential is called anode polarization. The main reason of anodic polarization is that the protective passivation film formed in the corrosion process prevents the direct contact between anode metal and solution, slows down the ion formation speed of metal, thus reducing the charge density on the anode surface and improving the electrode potential of anode. The change of cathode potential in the negative direction is called cathodic polarization. The main reason is that the cathode process consuming electrons is blocked, which makes the electrons of the cathode accumulate and increases the charge density on the cathode surface, thus leading to the cathode potential becoming negative. Because the anode becomes a positive electrode and the cathode becomes a negative electrode, the potential difference between the two electrodes is reduced, so the corrosion rate is slow. When almost all primary cells in stainless steel stop working, the material becomes a single-phase state. In other words, the battery on the first floor has only one pole and cannot form a loop. Here are the answers to your second and third questions.
3) The role of zinc plating should be mentioned in our high school chemistry textbook. The potential of zinc is lower than that of iron. In fact, chromium is also lower than iron. By sacrificing zinc and chromium, which are more active than iron, the potential of iron is increased, thus protecting iron.
As for the specific effect of chromium on the potential of iron, it was first studied by a scientist named Tammann. He found that when the Cr content in iron-based solid solution reached 12.5% atomic ratio (i.e. 1/8), the electrode potential suddenly jumped up. When the content of Cr increased to 25% atomic ratio (2/8), the electrode potential of iron-based solid solution suddenly increased. This phenomenon is called n/8 law of binary alloy solid solution potential, also called Taman law.