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081892B.CHM
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Country: India
From: Indian National Institute of Oceanography-Goa
1990-91 Annual Report p. 72-73.
MARINE CORROSION AND MATERIALS RESEARCH
(c) Corrosion studies (S.S. Sawant, N.B. Bhosle, K Venkat, A.
Nishad and Prabha Devi)
2) Effect of hydrostatic pressure on the efficacy of anti-corrosive methods
The following anticorrosive methods were used to assess their efficiency for
the protection of mild steel in deep oceanic waters: Cathodic protection by
sacrificial anodes - (i) Commercial aluminium-zinc anodes, (ii) Aluminium anodes,
coating of anticorrosive paint and conjoint use of paint and sacrificial
anodes.
The results after one year exposure of mild steel with these anticorrosive
systems in deep oceanic waters in the Bay of Bengal and Arabian Sea (1000 to
3000 m) revealed that the protection to mild steel by conjoint use of paint and
sacrificial anode was more effective than the other two types. The protection
was found to be almost 100% probably due to the double action of the system.
Paint coating (without sacrificial anode) also appears to have given fairly
good protection (99%). However, at greater depths (>1000 m) the paint film had
developed boils on the surface. Due to high hydrostatic pressure, the
seawater might have affected entry inside the paint film causing boils. Visual
observations revealed that the panels were corroded under the base of the
boils.
The extent of protection offered by sacrificial anode system was higher (90-
95%) at depths >1000 m. Out of the two types of anodes, aluminium was found to
provide better protection than the commercial Al-Zn anodes.
(d) Deterioration of concrete (K. V. Rao, S. S. Sawant and G.R. C. Aradthya)
1) Deterioration of concrete in marine environment
Studies on the deterioration of reinforced concrete have been carried out
with reference to alkalinity of concrete, diffusion of chlorides and sulphates,
XRD and SEM. The results indicated that the pH of concrete reduced from 12.5
to 9 and rate of diffusion of chlorides and sulphates increased with
increasing exposure time, leading to spalling and cracking of concrete.
2) Influence of dissolved salts on the corrosion of stressed RCC beams in the
marine environments
RCC beams of varying stresses were exposed to intertidal and total submerged
zones for assessing the corrosion of embedded steel with special reference
to the dissolved sea salts. The results indicated that the stresses of
varying degrees can permit the entry of salts such as chlorides and sulphates
by virtue of decreasing permeability of concrete. The amount of Cl- and SO4--
diffused into the concrete was comparatively higher at intertidal zone than
at other zones. This is mainly due to the wave aggressive action and
alternative wetting and drying of seawater on concrete structures.
Therefore, the rate of corrosion of embedded steel was higher at intertidal
zone.
1) Corrosion of metals and alloys in relation to the environmental parameters
To study the extent of corrosion, panels of mild steel (MS), copper, brass,
stainless steel (SS), cupro-nickel (Cu-Ni) alloy (90:1 0) were exposed for 15, 30,
60, 120, 180, 270 and 360 days at subsurface and near bottom depths at a
station in the coastal waters of Vijaydurg. The data obtained, indicate that
the metals and alloys corrode in their decreasing order as MS > Cu> Cu-Ni >
Brass > Stainless steel (Table 2). Stainless steel, though, found to be the
most resistant metal, was badly damaged due to pitting action. Corrosion rate
decreased with increasing period of exposure for all the metals. Corrosion
rate was higher at subsurface than that at the near bottom which can be
attributed to the higher concentration of dissolved oxygen at the former
depth.
Table 2 Corrosion rate (mdd) of metals and alloys exposed for different
durations in a coastal station in the west coast of India
Exposure Subsurface
period
(days) Ms Cu Cu-Ni Brass SS
15 120.2 53.7 49.8 20.5 2.9
30 98.9 36.0 27.6 13.1 1.9
60 74.3 26.2 18.2 8.9 8.4
120 49.0 11.9 12.8 5.7 5.5
180 47.7 11.0 10.5 5.2 4.3
270 58.7 7.9 8.9 4.2 3.5
360 67.7 8.9 5.9 0.9 2.0
Exposure
period Near bottom
(days)
Ms Cu Cu-Ni Brass SS
15 88.0 51.0 45.8 17.5 0.6
30 84.6 33.7 25.9 12.6 3.3
60 74.8 20.1 20.1 6.6 4.2
120 78.9 15.5 16.2 5.2 5.3
180 47.9 12.1 8.3 5.1 4.8
270 41.7 11.7 14.1 4.3 4.5
360 35.9 13.7 13.1 3.9 2.3
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