They are hot dip galvanized and still running.
There are three kinds of galvanized, hot-dip galvanized, mechanical zinc-plating, and electroplated. Each has its advantages and disadvantages from anti-corrosion performance, appearance, environmental pollution.
But if considering the anti-corrosion, costs, and long-term used, hot-dip galvanized is the best.
Hence, it is required in JG/T 503-2016 of Chinese Standard, ‘5.2.1Standard, ledger, diagonal brace, and accessories should be hot-dip-galvanized, painting or electroplated is not allowed.’
And some thickness requirements:
Galvanizing thickness (μm)
Steel tube thickness≥3mm, Local thickness: 55, Average thickness: 75
Steel tube thickness<3mm, Local thickness: 45, Average thickness: 55
Casting item, Local thickness: 60, Average thickness: 70