At approximately 15:40hrs, the IP had just completed a gas cutting task in the 25t line. The IP then proceeded to assist his co-workers who were installing U-strong backs to level the wall of the structure. This was done using wedges, U-strong backs and a sledgehammer to move the wall to final position for welding.The IP was part of a seven-man team (including foreman), three on gas cutting duties with the rest of the team fitting up the double bottom of Section 110, Block 20 with foreman present.When the IP had finished his duties on gas cutting, he made his way to the rest of the team and used a sledgehammer, hitting wedge, moving wall into place for welding. The U-strong back had been tack welded in two areas approximately 20mm on one side only. The correct process is to tack weld on both sides minimum 50mm.During this activity, the strong back broke away from the tack welds and hit the IP on the right shin resulting in a cut approximately 10mm in length. The strong back did have a lanyard fitted for dropped object prevention but was ineffective as the IP’s leg was close to the strong back when it broke free.The IP stopped work and informed his foreman what had occurred who informed Deeline HSE team member who then informed ATH HSSE supervisor.On investigation, it was found, the tack welds were non-compliant with ATH Tack Welding Procedure. The area where the tack welds were placed was also not prepared correctly resulting in welds not having good fusion. The tack welds (X2) were only 20mm in length and had only been welded on one side of the strong back. They should have been minimum 50mm and on both sides. The tack welds from previous use had also not been cleaned from strong back, which contributed to the lack of fusion on the tack weld.It was also noted that the IP was using a sledgehammer when a smaller hammer should have been used for this task.On further investigation, it was found that neither the foreman, tack welder or the IP checked the welds prior to using the hammer and wedge. All should have known the tack welds on the strong back were non-compliant and task should not have been undertaken.