Although United matched the home side for much of the opening half-hour, Kevin Phillips opener after 35 minutes and the sending off of Iron left-back Marcus Williams left them with it all to do.
Robert Koren doubled the home side's advantage on the verge of half-time, and United conceded three more times after the break.
"It was a great game, but I am very disappointed I didn't score," said Morrison, a summer signing from Premier League side Middlesbrough.
"It was another great home win and we are looking to make The Hawthorns a real fortress.
"It is vital that if we are going to stay at the top of the table we need to keep this good home form going."