Pollution rivals sea air for rust busting

Eskom and Sasol are among the heavy polluters contributing to an expensive rust problem in Gauteng, where some areas are now exposed to steel corrosion rates similar to those in coastal provinces affected by the salt-laden ocean - and in some cases even worse.

A new corrosion mapping project by researchers at Wits University - the first nationwide assessment in nearly three decades - has thrown new light on the potential for rust-related damage in several parts of the highveld due to corrosive industrial pollution from power stations, refineries and factories, and from airport and traffic fuel emissions.

Writing in the latest issue of the South African Journal of Science, Dr Darelle Janse van Rensburg of the Wits School of Chemical and Metallurgical Engineering estimates that about R166bn is spent every year to combat corrosion-related problems in SA - of which about R83bn is due to atmospheric (airborne) corrosion as opposed to internal or chemical corrosion in water piping and other infrastructure.

These calculations were extrapolated from a 2016 study by the Worldwide Corrosion Authority, which estimated the global cost of corrosion to be about $2.5-trillion (or R36.21-trillion, about 3.4% of global GDP).

According to Van Rensburg's study, the coastline near Melkbosstrand in the Western Cape is among the rustiest places in the country, with a mild steel corrosion rate of 460 microns per year. Other rust spots include Salisbury Island in Durban harbour (134 microns per year) and the adjoining Congella industrial area (122 microns).

In the Bayhead area of Durban, the mild steel corrosion rate of 66 microns is much lower, but similar to Empangeni on the KwaZulu-Natal north coast (58 microns).

At the more sheltered Ysterplaat air force base in Cape Town, the corrosion rate is just 28 microns.

Corrosion rates in several parts of Gauteng and neighbouring provinces are much higher - such as Germiston (51 microns), Sasolburg (50) and Secunda (38).

At the far end of the scale, the Karoo and other arid regions have the lowest risk of corrosion. Droerivier has a rate of just 1.3 microns and De Aar 2 microns.

Overall, Van Rensburg, professor Lesley Cornish and Dr Josias van der Merwe conclude that mild steel corrosion is worst in unsheltered stretches of coast within 150m of the sea - close to the surf zone where tiny bubbles of sea water burst into salty aerosols as they are blown inland.

Thereafter, corrosion rates drop by as much as 80% in the coastal zone between 150m and 1km from the sea.

This sharp drop in corrosivity, says Van Rensburg, is most likely due to airborne sea salt settling due to gravity and reduced air turbulence. Coastal dunes and thick vegetation along the KwaZulu-Natal coast also help to trap and intercept seaborne salt spray.

Further back from the coast, in areas between 1km and 3km from the sea, mild steel corrosion rates decline to "moderate" levels.

Surprisingly, however, corrosion rates increase markedly in parts of Mpumalanga, the Free State and Gauteng, despite being hundreds of kilometres inland.

Van Rensburg says the increased corrosion rate around Secunda and Emalahleni appears to be due to airborne pollution from industrial facilities, including the Sasol synfuel refinery, 11 Eskom coal-fired power stations and the many open-cast coal mines.

Sasol said "it takes note of the report and will study it further."

Higher steel corrosion rates are also noticeable in parts of the greater Johannesburg metropolitan area - especially the East Rand and southern Joburg. This appears to be driven by a combination of temperature, rainfall, mining and industrial pollution gases such as sulphur dioxide and nitrogen oxides.

Other sources of localised chemical air pollution include the Kelvin power station, Chloorkop industrial area, jet-fuel emissions from OR Tambo airport, gold-mine dumps and traffic emissions.

Eskom is already under fire from local community and environmental groups which claim that tiny dust particles and other toxic air pollution from its coal-fired power stations cause more than 2,000 premature deaths in SA each year, as well as thousands of cases of bronchitis and asthma.

The figures are based on a study by former World Bank and Greenpeace health impact consultant Dr Mike Holland, who calculated that these pollution effects cost SA more than R30bn a year through premature deaths, hospital admissions and lost working days.

R224bn 

The capital expenditure Eskom would need to spend to comply with minimum emission standards. according to the utility's 2016-2020 Air Quality Improvement Plan

Eskom, which has lodged several applications to be exempted from the government's minimum air emission standards, has previously acknowledged that "scientific studies confirm that Eskom's emissions are harming human health and causing premature deaths". This week when asked for comment Eskom did not respond.

However, according to the utility's 2016-2020 Air Quality Improvement Plan, complying with the minimum emission standards published in 2015 would cost Eskom R224bn in capital expenditure and a further R6bn annually in operating expenses.

Over the next few months, Van Rensburg hopes to shed more light on highveld corrosion rates by publishing finer-scale maps and more detailed data about corrosion levels at 57 test sites in greater Johannesburg.

According to similar corrosion studies in New Zealand, high levels of sulphur dioxide in the air can initiate chemical processes that lead to increased corrosion in building materials such as stone, concrete and metal.

The study, by the Building Research Association of New Zealand, notes that hydrogen sulphide emissions from fuel refineries and paper and food plants are also extremely corrosive to most metals, and sulphur and nitrogen emissions from vehicles generate highly aggressive acids.

Van Rensburg says corrosion monitoring in SA could, in some cases, provide an inexpensive tool to monitor the levels of air pollution. However, she stresses that the most recent published map, based on tests at more than 100 sites nationwide, focuses only on the corrosion of mild steel, whereas new South African corrosion maps for copper, aluminium, zinc and galvanised mild steel have also been developed.

SA's first atmospheric corrosion map (the "Callaghan map") was published 28 years ago. This was based on data collected from just 11 sites, most of which were on the South African and Namibian coastlines, and significantly overstated corrosiveness in some places. The new map helps to delineate more clearly SA's most corrosive environments.

As a result, it might be possible to use more environmentally friendly corrosion protection methods in areas at lower risk of corrosion. Currently, she says, the corrosion protection industry clearly distinguishes between the domestic, light-, heavy- and ultra-heavy industrial markets.

"Consequently, the use of water-based protective coatings, which are considered more user-friendly and environmentally friendly compared to most solvent-borne industrial coatings, are often overlooked in the light industrial market.

"Based on our research, it has been shown that when it comes to most inland areas, the biggest long-term enemy of organic coatings is sunlight radiation, rather that the corrosivity of the environment. Consequently, considering that many of SA's inland areas rate very low (C1) to low (C2) corrosive, the use of more environmentally friendly materials should be considered," she says.