Progressive Corrosion Study of Metals like Mild Steel , Zinc and Aluminium in an Urban Atmosphere

Corrosion rate (Progressive) of mild steel (MS), zinc and aluminum have been determined under outdoor conditions of exposure at Valsad (South Gujarat) representing an urban atmosphere. Mild steel (MS), zinc and aluminium plates exposed during November 2005 indicates corrosion rate of 88, 33 and 1.4 mg/sq.dm for one month exposure period and 3668, 968 and 26.1 mg/sq.dm for mild steel, zinc and aluminium respectively for twelve months exposure period. Mild steel panels exposed vertically suffer less corrosion than those exposed at an angle of 45°. The resistivity towards the atmosphere was in the increasing order: mild steel < zinc < aluminium. Corrosion rate of these three metals was found more in rainy seasons than the rate of winter and summer season.


Introduction
Urban atmosphere have pollution from domestic fuel consumption, exhaust of motor vehicles, etc. resulting mainly in an increase in sulphur dioxide, sulphuric acid and dirt with a slight increase in CO 2 and chloride.Urban atmosphere is similar to rural atmosphere where there is little industrial activity, characterized by pollution composed mainly of SO x and NO x variety, from motor vehicles and domestic fuel emissions which with the addition of dew or fog, generate a highly corrosive wet acid film on exposed surfaces (deposition rate of SO 2 higher than 15 mg m -2 day -1 and that of NaCl lower than this value).The present study was carried out in an urban atmosphere under outdoor exposure at Valsad situated in South Gujarat.
Twenty four specimens of each metal (mild steel, zinc and aluminium) were exposed at the same time (i.e. during November 2005).After completion of exposed period, the progressive weight loss of metal was determined.So, we get successive corrosion rates for one month, two months and three months up to twelve months.Similarly, another set was exposed during March 2006.
All tests were carried out in duplicate and mean of the two values were taken.After exposure period, test plates were wrapped in plastic bags and brought to the laboratory for cleaning.Different cleaning solutions were used to clean different metals.Clark's solution [1][2] were used to remove rust from mild steel made by 2% Sb 2 O 3 (antimony oxide), 5% Sncl 2 (stannous chloride) in concentrated HCl (100 mL) at room temperature with constant stirring about 15-20 minutes.Zinc plates were derusted by 10% CrO 3 and about 0.2 g BaCO 3 in distilled water (100 mL) at 25 °C for about 2 minutes 3 .Corrosion products on aluminium plates were removed by using the solution of concentrated HNO 3 containing CrO 3 (chromic acid, 50 mg/lit) at a room temperature for about 10 minutes 4 .

Meteorological and pollution data
The average maximum and minimum temperature was found about 310 K and 291 K respectively.Relative humidity was higher than the critical humidity value (70%) for three months in year 2006 and in 2007 (Figure 1).Generally, the rain starts in June and continuous up to October.The data of rain fall (in mm) and number of rainy days of the year 2006 and 2007 are mentioned in (Figure 2).Total annual rainfall was found 1540 mm in 2006 and 2064 mm in 2007.March, April, May and June are the hot months of the year, whereas December, January and February are considered as cold months.Max.  3).

Various exposure conditions
The corrosion rate of MS plates exposed to fully outdoor condition and partly sheltered condition from November 2005 to October 2006 indicates that the corrosivity is more in fully exposed condition (average 421 mg/sq.dm/month)than the partly sheltered condition (average 267 mg/sq.dm/month)(Table 1).

Zinc
The observation of first set of zinc metals exposed in winter months (i.e.November 2005) indicates that the corrosion rate increases progressively for the first four months (i.5).

Figure 1 .
Figure 1.Average relative humidity (R.H.) in percentage at an urban atmosphere.A sulphation rate was found in a range from 10.2 to 20.2 mg SO 3 /Sq.dm/month.A sulphation rate of 0.03 mg SO 3 /sq.dm/day(0.9 mg SO 3 /sq.dm/month) is usually accepted as representative of clean air 5 .

Figure 3 .
Figure 3. Progressive corrosion rate (CR) of mild steel under outdoor exposure at an urban atmosphere.
e. November 2005 to Febuary 2006) and then it steady for further three months(from March 2006 to May 2006) then it increased rapidly for further five months (i.e. from June 2006 to October 2006).The corrosion rate of zinc indicates 33 mg/sq.dmfor one month and 968 mg/sq.dmfor Twelve months exposure period.Another set of plates exposed in summer month (i.e.March 2006) indicates corrosion rate increases slowly for eight months (i.e. from March 2006 to October 2006), then it increased rapidly for further four months (i.e. from November 2006 to Febuary 2007) (Figure 4).

Figure 5 .
Figure 5. Progressive corrosion rate (CR) of aluminium under outdoor exposure at an urban atmosphere.
8ain fall (in mm) and number of rainy days at an urban atmosphere.The sulphation rates of different locations are observed as follows: 34.6 mg/m 2 .dSO 2 (average 6 months) at Cuba 6 , 0.3 to 9.0 mg/SO 2 /m 2 .d -f 22 urban atmospheric in the Ibero-American region 7 , 0.98 mg SO 3 /sq.dm/dayatJamshedpur8and0.85 mg SO 3 /sq.dm/dayat Chennai 8 .MS plates exposed in winter months (i.e.November 2005) indicates that the corrosion rate increases slowly up to seven months (i.e. up to May 2006) and then it increased rapidly for another five months (i.e. from June-2006 to October 2006).The corrosion rate of mild steel indicates 88 mg/sq.dmfor one month and 3668 mg/sq.dmfor twelve months exposure period.Similarly, plates exposed in summer months (i.e. from March-2006) shows corrosion rate increases rapidly for seven months (i.e. from March 2006 to September 2006), then it remain almost steady for further five months (i.e. from October 2006 to Febuary 2006) (Figure R.H, %Figure 2.

Table 1 .
Corrosion rate of mild steel in fully outdoor and partly sheltered condition at an urban atmosphere.The corrosion rate of aluminium indicates 1.4 mg/sq.dmfor one month and 26.1 mg/sq.dmfor Twelve months exposure period.Another set of aluminium plates were exposed in summer months (i.e. from March 2006).The corrosion rate increase rapidly for seven months (i.e. from March 2006 to September 2006), then it remain almost steady for further five months (i.e. from October 2006 to Febuary 2007) (Figure AluminiumAluminum plates exposed in winter months (i.e.November 2005) indicates that the corrosion rate increases slowly up to eight months (i.e. up to June 2006) then it increased rapidly for another four months ( i.e. from July 2006 to October 2006) (Figure5).