What is wrong with this code, does not work ?

Ian and BKBK both gave good answers.  In addition to what they said,

run a select query with the same where clause as your update query, and cfdump it and then cfabort your template.  Do you get any records?  If so, take out the cfabort, and do that select/dump thing before and after your update.  Did the data change?  If it did, what made you think it wasn't changing before?

It seems you want to get a comma-delimited list from form.delRecords. Then put the code <cfdump var="#form#"> in the action page. Experiment by submitting after switching the checkbox on and off. You will find that form.delRecords isn't the comma-delimited list you expect.

To get such a list you should have two or more form fields named delRecords. Even then, it is a shaky idea. Forms only submit checkboxes that have been checked.

Looks pretty good for the code you shown.  As usualy I would recomended one to use the <cfqueryparam ...> tag.

where serialNumberID IN (<cfqueryParam value="#form.delRecords#" list="yes" cfsqltype="cfsqlinteger">)

But that will functionally work the same as your code, just be much safer.

To actually debug your code, you need to see what is being output for the form controls, look at the source.  And see what is being returned to the sever in the form request, <cfdump var="#form#"> at the top of the action page.

Does the data look like it should?