The reduction mechanism of sulfuric and hydrochloric acids
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I am a young master’s graduate who has only just started working. In the field of chemical processing equipment, there are discussions and exchanges regarding stainless steel corrosion within the company. On the PowerPoint presentation prepared by one of my colleagues, it was stated that: \"Hydrochloric acid remains reducing throughout its entire concentration range, but the presence of oxidizing impurities can **alter its properties\"”； “Sulfuric acid exhibits slight reducing properties at room temperature when its concentration is below 20%; it has strong reducing properties when the concentration is below 60%, weak reducing properties at concentrations of up to 80%, and then begins to show oxidizing properties at even higher concentrations”… It really overturned my worldview! Since high school, it has been taught that hydrochloric acid and sulfuric acid possess oxidizing properties. Hydrochloric acid can exhibit reducing properties in certain situations due to the presence of chloride ions, but in sulfuric acid, both hydrogen and sulfur are in their highest oxidation states – so how can it have reducing properties? Is it oxygen? I asked my colleague, and he seemed to think I was from another planet; he cited words from an expert on a certain corrosion forum, and that’s indeed the case... The corrosion manual also states that hydrochloric acid is a strong acid with strong reducing properties... It also says that stainless steel forms a dense oxide layer on its surface, and since it corrodes in sulfuric acid or hydrochloric acid environments, hydrochloric acid and sulfuric acid have reducing properties – otherwise how could they react with the oxide layer to cause corrosion? Somehow I feel it’s not the same thing; isn’t the corrosion caused by hydrochloric acid and sulfuric acid due to chloride-induced stress corrosion or polythionic acid-induced stress corrosion? Is it the same as reacting with the oxide layer? So I’m hoping to get some help from the experts here – could someone explain it from a mechanistic perspective? 1. How can hydrochloric acid be explained as a strongly reducing acid, and what about the reducing property of sulfuric acid? 2. Is the corrosion of stainless steel caused by the breakdown of its oxide film, and are it are reducing substances that react with this oxide film? 3. By the way, why does concentrated sulfuric acid have strong oxidizing properties? What does \"strong\" mean in this context? Is it related to temperature? Thank you so much; it feels like the world has been turned upside down. . .
It seems I still need to study for a doctoral program.
It’s really tedious not to be able to insert pictures. Go and search on Baidu for redox reactions; those that lose electrons are called strong reducing agents
1. Hydrochloric acid is a strong acid with reducing properties, but its reducing strength is not high. 2. The oxide film is broken down, which leads to the classification of it as a reducing agent. This doesn’t hold water. Metal oxide films can be removed by more oxidizing agents, or they can also be dissolved. The passivation layer of metals is the result of a complex reaction; understanding it merely as an oxide layer is rather superficial. Maybe this professor is a fake professor ； Well, he attended the professor’s lecture, but only listened to half of it and wrote down one-third of it. 3. Concentrated sulfuric acid possesses strong oxidizing properties, and this oxidizing capacity is closely related to temperature. However, even below the freezing point, concentrated sulfuric acid still has a high degree of oxidizing power; it’s just that its oxidizing strength increases at higher temperatures! It also has a strong dehydrating effect. This dehydration does not refer to drying; rather, it means removing the hydroxyl and hydrogen atoms from the substances in contact, in the form of water molecules. Simply put, organic substances—especially sugars—carbonize. This is because the molecular formula of sugar consists of carbon atoms, hydroxyl groups, and hydrogen atoms, and the ratio of hydroxyl groups to hydrogen atoms is exactly 1:1, which corresponds to a water molecule. In another post, I replied with a presentation I created earlier, which covers topics such as corrosion and passivation. You can give it a try; if you really can’t find it, you can contact us again
You might not have read the post carefully. What I mean is that in sulfuric acid, both hydrogen and sulfur are in their highest oxidation states, and they should no longer have the ability to lose electrons; so I wonder how it can still exhibit reducibility. Redox reactions are still clear.
I also think he attended a class taught by a fake expert… Thank you very much. Let me check.