The concrete that coated the reinforced steel could not protect the steel from long exposure to oxygen in the air and presence of chloride in the sea water that will corrode the reinforced steel. Thus, corrosion protection need to be applied. Corrosion inhibitor is applied by layering the reinforced steel or the concrete with chemicals that could prevent Hydroxide ions and Chloride ions from reacting with the reinforced steel. Most inhibitors that are used in the building industry are harmful to the environment as they contain high concentration of heavy metals. Green corrosion inhibitors containing tannin which is obtained from plants has been proven to provide protection against the corrosion of the reinforced steel. Mangrove bark tannin is a potential green corrosion inhibitor as it is biodegradable, inexpensive, do not contain toxic compounds and utilises mangrove barks from the mangrove trees that have been cut down by the coal industries in Malaysia. © 2020 University of Manchester and the authors.