Atmospheric Corrosion of Carbon Steel in Mauritius B.Y.R. Surnam 1, Prof. Chetty V. Oleti 2 1 Phd Scholar at University of Mauritius, Faculty of Engineering, Mechanical and Production Engineering Dept. 2 Project Supervisor

Outline of presentation Introduction Introduction Methodology Methodology Results Results Analysis of results and conclusion Analysis of results and conclusion

Introduction Introduction MAURITIUS Tropical country Has a high humidity level. Has substantial rainfall. Is small and surrounded by sea. Expected high corrosion rate

The aims of this study are: To determine the corrrosivity of the Mauritius atmosphere through mass loss analysis. To determine the corrrosivity of the Mauritius atmosphere through mass loss analysis. To determine how the atmospheric corrosion of low carbon steel proceeds through surface roughness analysis. To determine how the atmospheric corrosion of low carbon steel proceeds through surface roughness analysis. To develop models for atmospheric corrosion in Mauritius. To develop models for atmospheric corrosion in Mauritius.

Methodology Material used: low carbon steel, medium carbon steel Material used: low carbon steel, medium carbon steel Size: 150 mm  100 mm  3mm Size: 150 mm  100 mm  3mm Sites: Sites: –Low carbon steel: Reduit, P.Louis, St Julien, Belle Mare –Medium carbon steel: Reduit, Vacoas, Palmar Specimens exposed over a time period of 1 ½ years. Specimens exposed over a time period of 1 ½ years.

At specific time intervals, they were removed and cleaned according to BS 7545 At specific time intervals, they were removed and cleaned according to BS 7545 Mass loss and average corrosion rate determined. Mass loss and average corrosion rate determined. Cleaned specimens have rough surfaces which varies with time of exposure. Cleaned specimens have rough surfaces which varies with time of exposure. Cleaned specimens were selected and 2D and 3D surface roughness measurements were taken using Form TalySurf Series 2. Cleaned specimens were selected and 2D and 3D surface roughness measurements were taken using Form TalySurf Series 2.

Selection of sites Exposure of samples Preparation of samples for exposure Mass loss & corrosion rate determination through weight loss analysis and electrochemical tests respectively Gathering of data for: Atmospheric parameters Pollution level Level of airborne salinity Removal of samples after specific time intervals Analysis of surface texture to understand corrosion process Models to determine and predict corrosion loss in Mauritius

Results Mass loss Mass loss

Surface roughness Days of exposure Average Mass loss(g) 3D surface texture of samples exposed at Reduit, after levelling and filtration

Days of exposure: 78.9Average Mass loss(g): Days of exposure: 391.9Average Mass loss(g):11.203

Analysis and conclusion According to ISO 9223 (1992), the sites at Belle Mare, St Julien d’Hotman and Reduit fall in category C 3,whereas the site at Port Louis falls in category C 4. According to ISO 9223 (1992), the sites at Belle Mare, St Julien d’Hotman and Reduit fall in category C 3,whereas the site at Port Louis falls in category C 4. Site Annual mass loss (g) Annual mass loss (gm -2 /year) Belle Mare St Julien d’Hotman Port Louis Reduit

Description of the corrosion process taking the roughness parameters into consideration. 1 st removal- Uneven general corrosion due to the presence of very porous rust layer on the metal surface 2 nd removal- Rust layer starts to become protective and corrosion proceeds through pores producing pits. 4 th removal – The pits grow in size but the metal at the top of the peaks are also consumed. So, they become shallower but larger. 3 rd removal- The pits continue to grow in size as corrosion proceeds through pores.

Modelling of atmospheric corrosion degradation results of mass loss Dependent variable (DV): Corrosion loss (obtained from mass loss) Dependent variable (DV): Corrosion loss (obtained from mass loss) Independent variables (IVs): Independent variables (IVs): –Time of exposure (days) for each set of samples removed at each removal. –The average daily values of the following atmospheric parameters corresponding to the exposure period of the samples: Relative humidity; Relative humidity; Rainfall; Rainfall; Hours of sunshine; Hours of sunshine; Wind speed; Wind speed; Temperature; Temperature; –24 hours average concentration of sulphur dioxide in the atmosphere. –The yearly average deposition rate of air borne salinity. –The carbon content of the carbon steel.

Model: ln (Depth of corrosion) = (Carbon Content) (Daily amount of sunshine) [ln (Days of exposure)] ln (Depth of corrosion) = (Carbon Content) (Daily amount of sunshine) [ln (Days of exposure)] R value = 0.93 R value = 0.93

Other models Electrochemical tests for only one site (St Julien): Electrochemical tests for only one site (St Julien): where C- corrosion loss (um/yr), t- time in hrs From surface roughness tests From surface roughness tests

To conclude: 3 models have been developed to predict atmospheric corrosion. 3 models have been developed to predict atmospheric corrosion. The corrosivity of the atmosphere, according to ISO 9223 (1992) and taking low carbon steel into consideration, in Mauritius was finally described as falling in: The corrosivity of the atmosphere, according to ISO 9223 (1992) and taking low carbon steel into consideration, in Mauritius was finally described as falling in: –Category C 4 for Port Louis. –Category C 3 for all the regions in the island apart from Port Louis.

Atmospheric corrosion proceeds when the rust layer is wet and decreases drastically, behaving like a protective coating, when it is dry. Atmospheric corrosion proceeds when the rust layer is wet and decreases drastically, behaving like a protective coating, when it is dry. The corrosion rate is very high initially (first 3 months) due to the unprotective, porous and thin rust layer. With time, it grows into a protective one and the corrosion rate decreases. Still, the carbon steel continues to corrode appreciably due to cracks formed in the rust layer such that the corrosion loss can be represented by the bilogarithmic equation. The corrosion rate is very high initially (first 3 months) due to the unprotective, porous and thin rust layer. With time, it grows into a protective one and the corrosion rate decreases. Still, the carbon steel continues to corrode appreciably due to cracks formed in the rust layer such that the corrosion loss can be represented by the bilogarithmic equation. The atmospheric corrosion attack and the development of the pores and cracks in the rust layer is a random process. The atmospheric corrosion attack and the development of the pores and cracks in the rust layer is a random process.

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