The paper describes the selection of materials for the reliable operation of oceanographic instruments. For selecting the material, raft immersion tests were carried out for one year. Results of the tests are presented. Comparisons between metals were discussed

Gopalakrishna Pillai, A.G.

Ravindran, K.

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Selection of s e Instruments 
A. G. GOPALAKRISHNA PILLAI and K. RA VIND RAN 
Central Institute of Fisheries Technology, Cochin - 682 029 
The paper describes the selection of materials for the reliable operation of oceano-
graphic instruments. For selecting the material, raft :immersion tests were carried out 
for one year. Results of the tests are presented. Comparisons between metals were 
discussed. 
Common oceanographic instruments for 
measuring temperature, salinity, depth, 
current direction and speed, dissolved oxygen 
and pH are prone to corrosion when exposed 
to seawater. Corrosion may seriously affects 
the electronic part of them also. The failure 
of the equipment can lead to catastropic 
happenings, both in cash and human reso-
urces. The environmental factors include 
high hummidity, variable temperature, 
salinity etc. all potentially determental from 
corrosion point of view. In order to com-
bat the highly corrosive environments the 
instruments should be made of corrosion 
resistant materials with high strength and 
of low specific gravity. Even though non-
metals are corrosion resistant, they have 
poor strength and fabricability. Because 
of the importance of the problem, the work 
was taken up as a part of the work under 
the comprehensive investigation on the corro-
sion behaviour of metals in Cochin Harbour 
(9°58'N, 76°16'E). 
Materials and Methos 
Carbon steel, stainless steel 304 and alumi-
nium alloy 5086 are the materials selected for 
the study, taking into consideration the cost 
factor, specific gravity and the physical and 
mechanical properties. The metals were 
cut to 10 x 75 cm from rolled sheets using 
guillotine with maximum care. As per 
ASTM (1974) method the metal plates were 
cleaned and weighed. The test panels were 
then mounted on mild steel racks of Carnegie 
Hlinois Steel Corporation design (LaQue, 
1948). The racks mounted with the metal 
plates were kept submerged at one metre 
level from the low water level in the Cochin 
Harbour. There were thirtynine panels of 
each of the metals tested. The microbiolo-
gical film formed on the metallic surfaces 
was removed at intervals using a very soft 
bristle brush with maximum care. The 
hydrographic parameters like temperature, 
dissolved oxygen, salinity and pH were 
determined during the period. The corro-
sion products on them were cleaned as per 
ASTM (1974) method and corrosion rate 
determined. 
Results and Discussion 
The corrosion rates of carbon steel, stain-
less steel 304 and aluminium HAL 5086 in 
the fouling free condition as a function of 
exposure periods are presented in Fig. l. 
The monthly variations of salinity, dissolved 
oxygen, surface water temperature and pH 
pertaining to this period of study were repor-
ted by Pillai. & Ravindran (1988). The 
principal factors which restrict the corrosion 
-
' 0 
600 
"'500 
E<posure period ( monm) 
Fig. l. Performance of materials. 
FISHERY TECHNOLOGY 
SELECTION OF MATERIALS FOR MARINE INSTRUMENTS 105 
and fouling on metallic panels is the peculia:r 
hyd:rographic feature of the Cochin Harbou:r 
(Pillai & Ravindran, 1983). The carbonsteel 
:registers rapid corrosion rate in the initial 
period of exposure of one to two months 
which gradually declined and attained a 
nea:r constant rate in a period of about 270 
days as can be seen from Fig. 1. The panels 
have high corrosion rate during the pre-
monsoon period but the rate was higher 
during the monsoon period and post-monsoon 
period. A comparable :result was :reported 
by Mishima (1940) who showed that mon-
thly cleaning of specimens had doubled the 
co:rrosion rate compared to fouled panels. If 
an annual corrosion rate of less than 0.13 mm 
and an annual pitting depth of 1.02 mm a:re 
considered in addition to corrosion aUowance 
when instruments are designed, no perfo-
ration will occur and reliable operation 
can be maintained during the designed 
life time (Jain, 1984). The study showed 
carbon steel undergoes uniform corrosion 
and comes to a uniform annual corrosion 
rate of 0.25 mm. No pitting corrosion was 
observed throughout the study. Carbon 
steel can be used only if protective measures 
are followed throughout its use. 
The corrosion of stainless steel was chara-
cterised by deep pits progressing into per-
forations. The panels remained free from 
pitting for about 30 days, but within a period 
of 2 months pitting and perforations were 
noticed. The mode of failure was in the 
form of long narrow tunnel. The perfora-
tions were in the direction of gravity, chara-
cterised by the adherent dense ferric chloride. 
The breakdown of passivity in pits occurs 
as the maintenance of passive film has not 
been possible in slow moving water at the 
test site (Paul & Moran, 1965). Much signi-
ficance cannot be attached to the co:rrosion 
rate of stainless steel based on weight loss 
as the metal loss is due to severe localised 
pitting and perforations. The localised 
attack is even more harmful than uniform 
Table 1. Analysis of variance of corrosion rate 
Source SS 
Total 100323.50 
Between metals 914715.44 
Between months 25402.30 
Error 71205.76 
Vol. 26, 1989 
df 
43 
3 
10 
30 
attack. While studying the corrosion beha-
viour of metals in Indian waters, De et al. 
(1968, 1977) observed pitting on 18:8 stain-
less steel below the base of barnacle and near 
the supporting holes. The present results 
show that the corrosion of type SS 304 was 
74 microns year- 1• As far as the perfor-
mances of type 304 are concerned neither 
the weight loss nor the occurrence of crevice 
attack gives a true picture of the resistance 
of the alloys to seawater. Results show the 
environmental factors influence considerably 
the service behaviour of this alloy. 
The corrosion rates of aluminium HAL 
5086 for various periods of immersion as a 
function of the pe:riod of exposure is given 
in Fig. l. In the present study extending 
to 360 days, no pitting, no crevice or no edge 
corrosion was noticed. This observation 
agrees, with those of W:right & Godard 
(1954). The graph shows an initial high 
corrosion rate followed by a slopping rate 
indicating the influence of the self repaired 
passive film, the thickness of which :reaches 
a limiting value which is influenced by the 
temperature, the dissolved oxygen, the pH 
of seawater and the ions present in the environ-
ment. De et al. (1968) also observed that 
the cor:rosion resistance of aluminium 
increased with the length of exposure. In 
view of the total absence of either pitting or 
crevice corrosion, corrosion rates based on 
weight loss measurements are meaningful. 
The uniform corrosion rate of about 17 
microns year-1 is observed at the end of 
360 days of tests. In the compilation of 
corrosion data of metals and aUoys by 
Schumacher (1979) corrosion rates of stru-
ctural metal and alloys at various sites of the 
world have been reported. The high degree 
of corrosion resistance and the absence of 
pitting of aluminium attest the cost effe-
ctiveness and usefulness of this material for 
making many sophisticated equipment. 
ms F 
304905.15 128.46** 
2540.23 1.07N.S 
2373.53 
106 A.G. GOPALAKRISHNA PILLAI AND K. RAVINDRAN 
Analysis of variance was carried out to 
compare the corrosion rates of carbon steel 
stainless steel 304 and aluminium 5086'. 
Significant difference in the average corrosion 
rates of different metals were observed 
(Table 1). 
The average corrosion rates of these metals 
are given in Table 2. Comparison amono-
means was carried by using the Least 
Table 2. Mean corrosion rate of different 
metals (microns year-1) 
Carbon steel 
SS 304 
Al 5086 
353.44 
30.96 
34.82 
Significant Difference (LSD) which worked 
out to 42.4. The average corrosion rate 
of carbon steel · exceeds this value from 
the averages of SS 304 and Al 5086 while 
the difference between the average corrosion 
rates of SS 304 and Al 5086 is far less than 
42.4. Therefore it can be concluded that 
while there is no significant difference in the 
average corrosion rates of SS 304 and Al 5086, 
the average corrosion rate of carbon steel 
is very high compared to those of the other 
two metals. 
The authors express their gratitude to Shri 
M.R. Nair, Director, Central Institute of Fisheries 
T~~nology, Co~hin for encouragements and per-
m1sswn to publish this paper. 
References 
ASTM (~974) Part 10, Designation Gl-72, 
Amencan Society for Testing and 
Materials, Philadelphia, 489 
De, C.P., Kelkar, V.M. & Vora, M.D. (1968) 
Proc. 2nd Int. Congr. Jl.far. Corros. 
Fouling, Athens, Greece, p. 55 
De, C.P., Chaudhuri, J.C. & Deshmukh, M.B 
(1977) Proc. Protection of Materials 
in the Sea, Bombay, India, p. 109 
Jain, C. (1984) Proc. 6th Int. Congr. Mar. 
Corros. Fouling, Athens, Greece, 165 
LaQue, F.L. (1948) In 'Corrosion Hand-
book' (Uhleg, H.H. Ed.) p. 1060, John 
Wiley & Sons, Inc., New York 
Mishima, T. (1940) Jap. Nickel Rev. 8, 90 
Paul, G.T. & Moran, J.J. (1965) in Corro-
sion Resistance of Metals and Alloys 
(LaQue, F.L. & Compson, H.R., Ed.) 
2nd edn., p. 375, Reinhold Publishing 
Corporation, New York, USA 
Pillai, Gopalakrishna, A.G. & Ravindran, K. 
(1983) Proc. 2nd Indian Conference in 
Ocean Engineering, Pune, India, p. 1062 
Pillai, Gopalakrishna, A.G. & Ravindran, K. 
(1988) Fish. Technol. 25, 83 
Schumacher; M. (1979) Seawater Corro-
sion Handbook, Noyer Data Corporation, 
New Jersy, USA 
Wright, T.E. & Godard, H.P. (1954) Cor-
rosion, 195 
FISHERY TECHNOLOGY 


Selection of materials for marine instruments

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Selection of materials for marine instruments

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