I have a text written in English (scientific). I want to ask somebody to give me some advice in writing and grammar. I have time to do the same for other persons in Spanish, my native language. Is there a site where I can "exchange" corrections?
It sounds to me like you're describing Lang-8.com, which describes itself as "A new language learning platform where native speakers correct what you write."
Although I have not used this service, and their introductory video makes it sound like it might be for more casual writing than something scientific, so YMMV.