In the raw .csv file the leading nulls are preserved (try opening it with Notepad and you will see), but when Excel reopens the file it converts them to numbers. But only those partnumbers that do not contain letters.

It is a clumsy solution, but you can rename your .csv to .txt. When you then open it you get to choose Delimited (next), Comma (next) and finally Column data format = text.

By the way, anyone know why my .csv files are actually semikolon delimited?