Metallurgy expert Dr Stephen Joel told Sheffield Crown Court the item had become corroded because it was not coated with sufficient zinc.

He also revealed the dangerous condition of the bracket - which supported the right-hand-side of the cradle - could have come to light before the tragedy which killed Sheffield painter Tony Bottomley, aged 60.

Colleagues Peter Wilson, of Burngreave, Anthony Glossop, of Manor Park, and Dale Howard, suffered broken bones and internal injuries.

Apollo Cradles Ltd, of Carlton, Barnsley, has pleaded not guilty to three offences under the Health and Safety at Work Act of not maintaining the equipment correctly.

The cradle was being used by workmen repainting the outside of the S1 St James Building, on Vicar Lane, Sheffield city centre, when the incident occurred in July 2003.

Dr Joel said: "The bracket had insufficient thickness of electro-plated zinc coating for outdoor use, which would have limited its ability to resist corrosion fatigue cracking."

He added: "Failure of the bracket occurred as a result of progressive corrosive cracking and, almost certainly, corrosion fatigue.

"This would have been present for some time and would have been detectable had the bracket been separated from the suspension form."

Jurors were earlier told the cradle and its supporting structure were left outside in all weathers for months before the collapse.

Witness Gareth Owen, a partner at a solicitors' firm based nearby described hearing a "loud scratching or scrabbling noise" as the cradle collapsed and seeing a "flash of white" when he looked through the window.

The commercial property lawyer said: "It turned out to be a man falling.

"I saw him hit the roof of the lower storey of the office block."

The trial continues.

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