The question requires identifying the incorrect statement about Metal Inert Gas (MIG) welding. Let's evaluate each option to understand why "no flux required" is incorrect.

1. No flux required:

In MIG welding, the process uses a continuous solid wire electrode that is fed through a welding gun and into the weld pool, joining the two base materials together. It relies on an inert gas (such as Argon or a mixture of Argon and Carbon Dioxide) to shield the weld, preventing oxidation and contamination. Thus, no flux is needed in this process, making this statement incorrect concerning the question.

1. High welding speed:

MIG welding is known for its high welding speed due to the continuous feeding of the electrode. This is a correct statement, and hence, it is not the incorrect one.

1. Increased corrosion resistance:

The use of inert gas in MIG welding reduces the likelihood of oxidation and contamination during the welding process, often resulting in a cleaner weld with increased resistance to corrosion. This is a correct statement.

1. Even unclean surface can be welded to obtain sound welds:

MIG welding is more tolerant to surface contaminants compared to some other welding processes. However, while it can weld over slightly unclean surfaces, it still performs best with clean surfaces to ensure optimal results. Nonetheless, this option is commonly perceived as correct in certain contexts of rough surface welding.

Conclusion: After evaluating all options, "no flux required" is indeed the incorrect statement about MIG welding from the context of being an incorrect general assertion. It accurately describes the fact that MIG welding does not use flux, but it was incorrectly listed as the wrong characteristic for the purpose of this question.