Not an expert but I do know many use ketchup on older coins to clean them.
The vinegar in it has some cleaning properties that can make some metal surfaces much better looking.
However I have never heard of anyone using this stuff to clean such a massive piece of rusty iron.
In your case you would need a ton of it to clean your knife...if it would even work at all.
You will probably get many more replies and I assume just about all of them will recommend electrolysis to clean that thing...just faster and way more thorough and efficient.
This is the relic guys process of choice to clean their better finds, weapon parts, old tools and whatever else pops up after having a very long dirt nap.
Your knife is a prime example of the kind of targets that will greatly benefit from this process.
Pretty cheap and easy to set up an electrolysis system, I know there are a few very good tutorials on this forum that will help you...just do a search and a ton of threads will pop up about this process.
Or, just get a wire brush attachment for a drill and have at it...that would probably work, also.