Electroless Nickel Phosphorus Coatings to Mitigate the Corrosion of Construction Steel

Introduction

Corrosion of construction steel is an important phenomenon which has gained global attention since it deteriorates lifetime of structures. This results in increased cost of replacement and maintenance globally (Liang et al., 2021). Rebars embedded in concrete are affected by attack of chlorides or sulphates and they appear as pitting or spalling (Duarte et al., 2014; Deus et al., 2014). Initially, there is a protective passivating layer on the rebar embedded in concrete in alkaline environment (Shi and Ming, 2017). But higher chloride concentration breaks through this passivating layer and cause further damage to the structure. Usually, austenitic stainless steel is considered to have superior corrosion resistance and are recommended in marine environment (Duarte et al., 2014). Other recommended grades of steel for construction which possess high corrosion resistance include duplex stainless steel (Alonso et al., 2019), Cr modified HRB400 steel (Liu et al., 2015), thermomechanical treated rebars (Torbati-sarraf and Poursaee, 2018), etc.). In fact duplex stainless steel was also seen to be passive with the formation of a protective oxide layer. Duplex stainless steel with lower Ni demonstrated lower corrosion resistance in presence of chlorides and pitting corrosion was detected in the rebars over a long time period (Alonso et al., 2019). Molybdenum enhances resistance to initiation and propagation of pitting and crevice corrosion. Stainless steel rebars are prepared with the strength and dimensional properties considering the requirements of structural concrete codes of practice. The British Standard Specification is BS 6744:1986 austenitic stainless steel bars and lists alloys such as 304, 304L, 316, 316L, etc. Austenitic steel is resistant to corrosion in concrete with very high chloride content, and is the recommended material for rebars. Luo et al. (2017) considered the electrochemical and passivation behavior of 316L stainless steel in chlorinated simulated concrete pore solution. It was seen that the solution pH had a significant influence. Formation of molybdates in the outer layer of passive film further enhanced the corrosion resistance. The presence of high Cr/Fe ratio in the passive layer also resulted in nobler corrosion potential.

Recent trends indicate the application of coatings and surface engineering of rebars to increase the working life of structures. Epoxy and enamel coatings are the most widely used variants in the construction industries (Tang et al., 2016a). Enamel coating was found to be the most suitable candidate over a long duration (Tang et al., 2016a). Though, careful handling of enamel coatings is required during transportation to prevent damage. Epoxy coating on the other hand show good bonding capabilities when embedded in mortar (Pour-Ali et al., 2015). Epoxy coatings on the other hand have the disadvantage that it fails to prevent corrosion expansion in case the defect to rebar area is greater than 5% (Zhicheng, 2016). Crack width in epoxy coated rebar could be reduced by addition of polypropylene fiber. Graphene modified epoxy coatings also provided excellent corrosion resistance (Sharma et al., 2022). A duplex enamel-epoxy coating resulted in an increase in corrosion resistance (180%) (Tang et al., 2016b). Recent studies have also reported the role of nano-fillers in epoxy coatings (Ranjitha et al, 2020; Khodair et al., 2019). Corrosion mitigation in rebars could be also achieved from soy-protein and corn-derived polyol coatings (Sajid et al, 2022).