Indian Journal of Chemical TechnologyVol. 1, September 1994, pp, 279-282

Cathodic protection of mild steel and copper in deep waters of theArabian Sea and Bay of Bengal.

S S Sawant, K Venkat & A B Wagh

National Institute of Oceanography, Dona Paula, Goa 403004, India

Performance of cathodic protection system to mild steel and copper in deep (> 1000 m) oceanicwaters of the Arabian Sea and Bay of Bengal has been assessed using aluminium and mild steel sac-rificial anodes. The corrosion rates of unprotected metals as well as their percentage protection in-creased with increasing depth. The ranges of protection achieved have been found to be 74-91%and 92-97% at depths of 1000 and 3000 m respectively. The difference in corrosion rates and thedegree of protection of the metals at different depths have been attributed to the concentration ofdissolved oxygen at those depths.

Though the use of cathodic protection and metal-lic coatings is believed to be the most effective forcontrolling corrosion in marine environment, theirefficiency depends on factors like water currents,temperature, salinity, dissolved oxygen, pH, andbiofouling', which are site specific. A number ofsacrificial anodic materials are being used for ca-thodic protection of metals, e.g., mild steel, is pro-tected against corrosion by coupling with alloys ofmagnesium, zinc and aluminium, whereas, coppercan be protected with metals such as mild steeland aluminium", Information on the corrosion be-haviour of metals and alloys and their cathodicprotection in the oceanic waters is scanty':", Thepresent investigation has been, therefore, under-taken to augment the knowledge about factorswhich would help in developing efficient technol-ogy for exploitation of deep sea resources. Corro-sion rates for cathodically protected and unpro-tected mild steel and copper have been estimatedby exposing them in deep oceanic waters below1000 m for one year and is discussed in relationto important physico-chemical parameters.

Experimental ProcedureCommercial grade mild steel (C - 0.2%,

Si - 0.5%, Mn - 0.26%, P- 0.05%, S - 0'()4%, Fe-rest) and copper (Ni - 0.05%, Cu-rest) panelswere used. Each metal was tested in two differentsets, one cathodically protected and the other un-protected. For achieving cathodic protection, mildsteel was galvanically coupled with commercialgrade aluminium (Si - 0.15%, Mn - 0.8% and AI-rest) whereas, copper was coupled with mild steeland aluminium separately. The cathode to anode

area ratio was 10:1 for all the samples. The pan-els and sacrificial anodes were cleaned followingASTM standard procedure and weighed beforeimmersion". Two stations each were selected inthe Arabian Sea and Bay of Bengal (Fig. 1). Thepanels were exposed at different depths at eachstation with the help of deep sea mooring systemas described earlier". The depths of immersionwere 1100, 1700 and 2300 m in northern Bay ofBengal (site 4) and 1000 and 2300 m in the cen-tral Bay of Bengal (site 3). In the Arabian Sea thedepths were 1100, 2100 and 3000 m in westernArabian Sea (site 2) and 1700 and 2900 m ineastern Arabian Sea (site 1). The panels were re-trieved after one year (Nov. 89-Nov. 90) cleanedand weighed. The loss in weight of each paneland the anode were recorded. The corrosion ratehas been expressed as mils per year (MPY). Thepercentage protection of metal was calculated us-ing the following formula:

Fig. I-Station location map

280 INDIAN J. CHEM. TECHNOL., SEPTEMBER 1994

. 100 (x- y)% protection = -----'-----'--'-x

where x = corrosion rate of unprotected metaly= corrosion rate of protected metal

In addition, water samples were collected fromthe depths under study for determination of salin-ity, pH and dissolved 'Oxygen at the time of de-ployment and retrieval of panels. Reversing ther-mometer was used for recording water tempera-ture.

Results and DiscussionPhysico-chemical parameters- The physico-

chemical parameters were found to vary withvarying depth. The water temperature in Bay ofBengal varied from 2.1°C at 2300 m to 6.4°C at1000 m whereas, it ranged from 1.6°C at 3000 mto 5.4°C at 1100 m in the Arabian Sea. The sali-nity ranged between 34.84 ppt .at 3000 m to35.37 ppt at 1100 m depth in the Arabian Seawhereas, it varied from 34.76 ppt at 2300 m to34.93 ppt at 1000 m in the Bay of Bengal. Theconcentration of the dissolved oxygen in theArabian Sea varied from 0.56 mUL at 1100 m to3.18 mUL at 3000 m whereas, in the Bay ofBengal it varied from 1.0 mUL at 1000 m to 2.9mUL at 2300 m. The pH ranged between 7.8 at1100 m and 7.92 at 3000 m in the Arabian Seaand from 7.68 at 1100 m to 7.72 at 2300 m inthe Bay of Bengal.

Mild steel-The corrosion rate of unprotectedmild steel panels increased with increase in thedepth of immersion (Fig. 2). These panels werefound to suffer with general type of corrosion.The corrosion rates ranged from 1.6 MPY at1100 m to 2.1 MPY at 3000 m depth in theArabian Sea and between 1.56 MPY at 1000 mand 2.3 MPY at 2300 m in the Bay of Bengal,i.e., the rate increased by - 31-47%. The dis-solved oxygen content was also found to increasewith increase in depth. Similar results were foundin an earlier study:'. The results thus show a closerelationship (coefficient of correlation r= 0.87 to0.90) between the corrosion rate and dissolvedoxygen concentration. Coefficient of correlationwas calculated using following formula:

C ffici f . () ~xy- myoe cient 0 correlatIon r = ---no, a,

_ LXx=-

n

- ~yy=-n

2 ®>Q.1·6:Ii

~ 1·20..c: o·0

••0 0'4t0u 0

2·4- -©-- r--

--;-

.-~ t7A . .P7I

z.o>Q.

:E 1·6

...• 1·20..

0·8c:o~ 0·4....

«3 01100 1700 2300

Depth. m

- ®- ----

-~ ~

1700 2900Depth. m

- -@-

- ----

~ ~1000 2300

Depth, m

Fig. 2- Variation of corrosion rates of cathodically unprotect-ed (0) and protected (.) mild steel at different depths inwestern (A) and eastern (B) Arabian Sea and in northern (C)

and central (D) Bay of Bengal

_J~ /(/y- yia;», n

wherex= value of dissolved oxygen concentrationy= value of corrosion raten = number of values

On the other hand the corrosion rate of ca-thodically protected MS below 1000 m was foundto increase with increase in depth (Fig. 2) andranged between 0.14 to 0.34 MPY in the ArabianSea, and between 0.12 to 0.44 MPY in the Bay ofBengal. The trend of the corrosion of protectedpanels was just reverse to that of unprotectedpanels, i.e., the percentage protection of metal wasfound to increase with increased depth. A mini-mum of 75% protection was observed at 1000 mdepth and a maximum of 94% protection was ob-served at 2300 m depth in the Bay of Bengal. Inthe Arabian Sea the values were 81 and 93% at1100 and 3000 m, respectively. The difference in

SAWANT et 11/.: CATHODIC PB.OTEClION OF MILD STEEL AND tOPPER 281

CBBT 2

©

Oepth.m.

A

©

3000

Depth, m

1.100 2100Depfl1. m

~ 0·8:l;~0·6"

o\.. 0-4c:.!!

e 0'2koU 0

Fig. 4- Variation of corrosion rates of cathodically unprotect­ed (0) and protected (with MS-anode rt2 and AJ-anode .)copper at different depths in western Arabian Sea (A), central(B) and northern (C) Bay of Bengal and eastern Arabian Sea

(D)

tive oxide film. However, the present study indi­cated that aluminium anode underwent higher dis­solution at greater depth. The possible explana­tion to this effect could be given based on thehypothesis proposed by Deltombe and Pourbaix6•According to them aluminium gets oxidised inwater by followingreaction.

2Al +JH20-+ Al203 + 6H+ + 6e~E= -1.494 - 0.0591 pH

One H + is formed for each electron. If the hy­drogen ions accumulate, the pH become very lowand the protective oxide filmwould get dissolved.

Copper-Like mild steel unprotected copperpanels also exhibited increase .of corrosion rate atdepths greater than 1000 m (Fig. 4). General typeof corrosion was observed in all the panels. Theincrease may be attributed to the higher concen­tration of dissolved oxygen. The corrosion ratesin the Bay of Bengal varied from 0.24 to 0.63MPY whereas, in Arabian Sea the rates rangedbetween 0.33 to 0.67 MPY, i.e., the corrosion rateat greater depth increased by 103-162%. Theserates were 3-7 times lower than that of MS. Thecorrosion rates for cathodically protected copperwere found to range between 0.015 to 0.04 MPY,irrespective of the type of anode, i.e., approxi­mately 92-97% protection was achieved. LikeMS, the protection of copper was found to in­crease with increased depth. Both MS and Alanodes were found to perform well and providedalmost the same degree of protection to copper.In this case also the increase of protection of ca-

,..0'8"-~~0'6..

<;

k 0'4.,o

~ 0-2

8 0

... (ii)

Earlier3 it was observed that the corrosion rateof unprotected aluminium decreases with increaseof immersion depth in deep oceanic waters. Thedecrease of corrosion rate at greater depth couldbe attributed to the higher amount of dissolvedoxygen which is responsible for forming protec-

Fig. 3-Photograph of AJ anode on mild steel showing flakycorrosion product

corrosion rate of protected MS at various depthsmay be attributed to the difference in the extentof loss of anode material. The aluminium anodesas can be seen in the Fig. 3 produced a adherentflaky product which was milky-white and coveredthe whole anode surface. The increase of percen­tage protection below 1000 m is associated withthe increase of per cent loss of anode materialand dissolved oxygen concentration (Table 1). It istherefore, assumed that the oxygen which is theimportant cathodic reactant as per the reaction (ii)might have promoted the cathodic reaction atgreater depth, thereby, causing greater loss ofanode and hence higher protection to thecathode.4Al-+4Al3++ 12e-

(reaction at anode i.e.,at Al) ... (i)

6H20 + 302 + 12e- -+120H­(reaction at cathode i.e.,at MS)

282 INDIANJ. CHEM. TECHNOL .. SEPTEMBER 1994

Table 1- Variation of physico-chemical parameters, percentage cathodic protection and percentage weight loss of anode mate-rial at various depths in deep oceanic waters

A B C D E F G H I J K L

Site 1 1700 3.25 35.00 2.15 7.84 82 11 93 31 93 252900 1.72 34.83 2.99 7.89 88 12 95 41 96 53

Site 2 1100 5.40 35.37 0.56 7.80 81 9 95 23 95 482100 2.50 34.95 2.70 7.87 91 16 96 76 92 643000 1.60 34.84 3.18 7.92 93 18 97 78 97 66

Site 3 1000 6.40 34.93 1.00 7.68 75 10 92 32 92 182300 2.25 34.76 2.90 7.72 89 16 94 66 93 38

Site 4 1100 5.91 34.90 1.50 7.68 75 12 93 22 93 291700 3.19 34.82 2.40 7.70 92 11 96 58 93 382300 2.10 34.78 2.90 7.72 94 17 97 77 97 46

A: Exposure site; B: Depth(m); C: Temp (0C); D: Salinity (ppt); E: Dissolved oxygen (mLL - I );F: pH; G: % protection by AIanode to MS; H: % AI anode wt. loss on MS; I: % protection by MS anode to copper; J: % MS anode wt. loss on copper; K: %protection by AI anode to copper; L:% AI anode wt.loss on copper.

thode was associated with the increase in percen-tage weight loss of both type. of anode materialsand dissolved oxygen concentration. This showsthat better the dissolution of an anode materialhigher the protection 7.

ConclusionTherefore, it can be concluded that both alumi-

nium and mild steel anodes performed well andequally for the protection of mild steel and cop-per at greater depths. Aluminium theoreticallypossesses an adequate potential and very highcurrent output for use as sacrificial anode. How-ever, the utility of pure aluminium has not yetbeen realised successfully because of a protectiveoxide film formation on its surface". Moreover,corrosion rate of unprotected mild steel and cop-per increased with increase of immersion depthbelow 1000 m whereas it decreased with increaseof immersion depth below 1000 m for protectedmild steel and copper. Dissolved oxygen appearsto be one of the important factors for this change.A linear relationship was observed between theloss of anode materials and the amount of protec-tion to cathode.

AcknowledgementsThe authors are grateful to the Director, NIO

for constant encouragement. Thanks are due toMr R R Nair, Project Co-ordinator, INDO-FRGProject and Dr V Ittekkot of Geoloish-Palaonto-loishes Institute University, Hamburg, FRG forproviding necessary mooring facilities. Sincerethanks are also due to Mrs A Garg for help in an-alysis of samples.

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Wiley & Sons, New York), 1975, 209.2 Ashworth V, Corrosion; industrial problems, treatment and

control techniques, (KFAS Proceedings Series, PergamonPress, Oxford, New York), 13(2) (1987) 1.

3 Sawant S S & Wagh A B, CO"OS Prev Control, 37(1990) 154.

4 ASTM Standard Methods for Chemical Cleaning after Test-ing, 1978,41.

5 Sawant S S, Venkat K & Wagh A B, Indian J Technol, 31(1993)862.

6 DeItombe E & Pourbaix M, Corrosion, 14 (1958) 496.7 Yunlong Zhu & Guangzhang Chen, Proc Int Conf on Cor-

ros CO"OS Control for Offshore and Marine Constr, Xia-men. China, 1988, 325.

H Ashworth V & Booker C J L, Cathodic protection: Theoryand practice (Ellis Horwood Ltd, England), 1986,83.