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Are all corrosives either acid or a base? From what I could remember from school, strong acids and bases can be corrosive, but can substances be corrosive from reasons other than that? I tried my best to check for the answer with my limited knowledge.

If it is not just acids and bases what is it then, how can we categorize them, if possible in simple terms?

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    $\begingroup$ Almost (if not all) all acids and bases are corrosives but all corrosives are not just acids and bases. $\endgroup$ Jun 7 '20 at 3:10
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Wikipedia lists many corrosive compounds. While a lot of them function as strong acids or bases, there are also these (not an exhaustive list):

  • Strong oxidizers including concentrated hydrogen peroxide
  • Fluorides (they say "fluoride ion", so salts as well as the acid are meant)
  • Organic compounds that can act as alkylating agents such as methyl sulfate

The moral of the story: Hazards can't be pigeonholed into specific types of chemicals. Best to find and read the safety data sheet with any chemical.

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    $\begingroup$ I detect two downvotes, in both this answer and another one that previously had a positive score. Oh boy are we having a bad day. Anyone care to explain? $\endgroup$ Jun 7 '20 at 14:33
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    $\begingroup$ Besides seawater, as described below, I am unsure how the 3 mentioned examples act as corrosives? (: Why are they corrosive? $\endgroup$ Jun 8 '20 at 13:29
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    $\begingroup$ @AndreasJørgensen, Look up the definition of corrosion provided by IUPAC. I think that you are thinking of corrosive nature from human safety point of view. That is not the typical meaning of corrosion in surface chemistry or material science. $\endgroup$
    – M. Farooq
    Jun 8 '20 at 14:56
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Operatively, the most corrosive substance on planet earth is likely seawater (which has a pH of 7.5 to 8.4). Ordinary pure aqueous NaCl also does a very good job on corroding Aluminum alloys, being able to attack the protective Al2O3 coating.

Part of the explanation is likely due to the ability of NaCl to be an excellent electrolyte serving in a variety of galvanic corrosion reactions. The chloride anion is likely a factor given its ability in forming complexes and being able to act as a reducing agent.

Also, with respect to water per a source:

Water is the enabler of fast oxidation of iron so freshwater will also cause rust. However, salt water is a very good conductor (lots of dissociated ions) and so there are a number of electrolysis reactions that tremendously accelerate corrosion in salt water.

Also, more on water per another source, to quote:

This means the water is donating the hydrogen ion, which classifies it as an acid using the Brønsted concept. ... As these two reactions show, water can act as an acid or a base; molecules (or ions) that can do this are called amphiprotic. When an amphiprotic molecule (or ion) reacts with an acid, it acts as a base.

As to the question "Are all corrosives either acid or base?", considering the case of water, the 'or' part of the question may be problematic.

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It is an open ended question. What is meant by corrosion to begin with? Corrosive is very general term which literally meant to "gnaw into pieces" like the worms.

Do you know pure supercritical water is quite a corrosive substance. It will attack many metals. Others have given you examples. Fluorine gas is corrosive. Who said that only acid and bases can corrode materials?

Formally, from IUPAC corrosion is "an irreversible interfacial reaction of a material (metal, ceramic, polymer) with its enviroment which results in consumption of the material or in dissolution into the material of a component of the environment. Often, but not necessarily, corrosion results in effects detrimental to the usage of the material considered. Exclusively physical or mechanical processes such as melting or evaporation, abrasion or mechanical fracture are not included in the term corrosion."

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