To combat corrosion of galvanised steel regular maintenance is required, regalvanising and painting are performed to extend the products life. When looking at the price of an asset it is important to take into account all the costs associated, throughout its life – initial, operating, maintenance, replacement and disposal. This is the assets Life-Cycle Cost (LCC). The maintenance and servicing associated with galvanised steel incur costs over time, which increases its LCC.

The addition of alloying elements such as chromium and nickel often leads stainless steel to having a higher initial cost. However, LCC calculations have been performed to determine which metal is the most cost effective over time.

A review published on Water UK provides some examples of different applications of stainless steel and galvanised steel within the water industry and a comparison of their life cycle costings (please refer to the reference list). One of these examples is shown in figure 2, revealing that after less than 5 years (green arrow) a galvanised steel access cover is more expensive than its stainless steel equivalent cover. The reason for this is regular maintenance and servicing (regalvanisation, painting) of galvanised steel, to preserve its corrosive resistance, incurs costs that add to its overall LCC. In addition, this study takes into account that a galvanised steel cover is replaced after 25 years, whereas a stainless steel equivalent will last longer than 50 years, another factor that is considered by LCC calculations. The lower life cycle costing of stainless steel is a significant benefit and one that is most often overlooked within the water industry.

Corrosion in Drinking Water Systems

Within the UK a wide range of metals are used in association with potable water systems such as pipes, access covers, water-tight doors, storage tanks and so on. Galvanised steel is popular within potable water systems due to its low initial cost and corrosion resistance. However, it is commonly seen that the zinc coating on galvanised steel products is below the European standard due to corrosion by, chlorine and sulphate concentrations, pH and temperature elevations (Delaunois, Tosar and Vitry, 2014). Stainless steel will still be susceptible to these factors. Yet, due to stainless steel not having a protective coating and its corrosive resistance being throughout the metal, this problem is only associated with galvanised steel.

When looking at figure 3, which was taken from a study by Allion et al., (2011), it clearly reveals that galvanised steels corrosion rate is far higher than stainless steel within potable water systems. Other studies have shown this clear difference in corrosion rate which can lead to leaching and leaks (Lee, Rasheed and Kong, 2018). Leaching of metals into potable water could be problematic depending on the constituent elements of the zinc coating (some older galvanised pipes may contain lead and cadmium). The higher corrosion rate also means more maintenance and eventual replacements are necessary for galvanised assets and not for stainless steel (Figure 4 and 5).

Both preserving water quality and maintenance of assets have associated costs and in turn, add to galvanised steels Life-Cycle Cost.

A comparison in wastewater

Wastewater treatment plants typically suffer from frequent and accelerated corrosion due to the extremely harsh environment subjected on the metals used. Depending on the location of the wastewater plant, various types of corrosion are common.

Areas of high flow rate experience flow-induced corrosion and erosion-corrosion. A typical example is screening, in this scenario, the equipment used needs to be able to withstand highly concentrated chemicals and abrasive particulates such as grit. As noted earlier, zinc is a soft metal that is easily damaged. In these harsh environments, the zinc coating on galvanised steel will be removed quickly exposing the unprotected mild steel beneath (Figure 1a). In comparison, stainless steel is much harder which reduces erosion-corrosion. Chromium has a hardness of 8.5 Mohs compared to Zinc at 2.5 Mohs (Tabor, 2007). This significantly increases the hardness of stainless steel making it much more resistant to abrasion compared to galvanised steel.

When looking at the comparison of sludge screens in figure 6, the pictures reveal no corrosion on the stainless steel screens after 12-13 years whereas the coated carbon steel equivalent screens reveals extensive corrosion after 12 years and required total replacement. The maintenance and replacements required due to corrosion incur costs and will add to its Life Cycle-Costing (LCC). It is more beneficial to invest in stainless steel, as over the LCC significant savings will be made.

In areas of low flow rate pitting corrosion is more common. Pitting corrosion is where a localised area of material is exposed to concentrated solutions or microbes for an extended period causing cavities and eventually holes. Pitting corrosion is problematic for both galvanised steel and stainless steel. The fast nature of pitting and reduction of oxygen means the passive layer for stainless steel cannot reform (Corrosion Handbook, 2015). In environments where pitting corrosion could be an issue it is important to consider the use of stainless steels with a higher molybdenum content such as duplex (British Stainless Steel Association, 2002). The variety of stainless steels makes them a great material to deal with the range of extreme environments found within the wastewater industry.