The 25-year-old joins until January, and becomes Stuart McCall’s first addition since taking over as Iron manager.
Alnwick started his professional career at Newcastle United, who he had joined from Sunderland. He appeared for the Toon Army in the Premier League and made eight appearances in all competitions.
While at St James’ Park, he enjoyed loan spells at Gateshead and Bradford City, before joining Port Vale on a free transfer in 2015.
Alnwick was at Vale Park for 18 months, making 79 appearances in all competitions. He then switched to Scottish Premier League giants Rangers for an undisclosed fee in January 2017. He has made 11 appearances in all competitions for the Gers.
The goalkeeper will take the Number 25 shirt.
