I had a similar issue where my string fields were being converted to numeric fields when I opened my downloaded data in Excel. This is due to how Excel is interpreting the data.
I think this community post, String Format Changing, pretty much covers the options for how to avoid this.
Depending on how you are using the data, another option could be changing the format or adding a non-numeric character to your Articulo field, so when you open the data in Excel, it will recognize it as a string. Some ideas can be found here, https://superuser.com/questions/318420/formatting-a-comma-delimited-csv-to-force-excel-to-interpret-value-as-a-string.