corrosion behaviour if ni-resist cast irons in seawater

CORROSION BEHAVIOR OF NI-RESIST CAST IRONSIN SEAWATER1

A.U. Malik, S. Basu, Ismail Andijani and Shahreer Ahmad

SUMMARY

The corrosion behavior of Ni-Resist cast irons in seawater has been studied under varyingtemperature and dissolved oxygen conditions. The studies involve investigations on thefailure of a Ni-Resist cast discharge column of a brine recycle pump from a desalinationplant and laboratory tests on two types of Ni-Resist irons. Weight loss measurements,electrochemical polarization techniques and metallographic methods of analysis have beenused to determine the corrosion rates and to investigate the nature of corrosion in Ni-Resistirons.

It has been established that discharge column of the brine recycle pump has failed by stresscorrosion cracking (SCC). The cracks are initiated from the pores and the cracking patternis characterized by the emanation of fine cracks from big cracks. The region near theweld/base metal interface has been found to be a preferential site for the initiation andpropagation of SCC due to presence of higher concentration of chromium carbide.

The corrosion rates of Ni-Resist T2 iron having graphite flake structure are higher than thecorresponding 02 alloy having graphite nodule structure. The corosion rates of Ni-Resistirons are strongly dependent on the dissolved oxygen. The corrosion rates are minimumin deaerated conditions although no passive region is observed in the potentiodynamicpolarization and open circuit corrosion potential plots. Under fully aerated conditions,only a region of limited passivity is invariably observed because of the poor adherence ofthe oxide scales.

1. INTRODUCTION

Ni-Resist cast irons are high nickel chromium containing austenitic cast irons usedprimarily for their corrosion resistance especially in sea water environments. Thesealloys tend to maintain their corrosion resistance under high velocity conditions andshow good wear resistance. Ni-Resist cast irons are the most frequently used materialsfor components handling sea water and brine such as large intake, recycling and blowdown pumps for desalination and power plants. Pitting of Ni-Resist type 2 in deaeratedsea water is minimal and therefore these materials provide excellent cover to stainlesssteel components against pitting and crevice corrosion during shut downs.

1 Issued as Tech Report No. SWCC (RDC)-12 in October, 1991

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The graphite in Type 1 through 5 in Ni-Resist iron is in the form of flakes as is commonin the structure of gray cast irons. It has been reported’ that austenitic flake graphitecast irons are more resistant to graphitization because the potential difference betweenmatrix and graphite is less than between the ferrite in cast iron and graphite. Thecorrosion rate of Ni-Resist types 1,2 and 3, showed an average corrosion rate of 1.6 milsper year (mpy) in quiet seawater, and 2 and 3 mpy at flow rates of 5 and 27 fps,respectively. In deaerated sea water, the corrosion rate of Ni-Resist Type 2 was inde-pendent of velocity but in aerated sea water, the corrosion and pitting rates are strongfunctions of velocity2. The pitting rate of Ni-Resist Type 2 is related to velocity by :

mpy (pitting) = 210 (v)0.37

In spite of their good service performance in desalination plants, there are some reportedcases of Ni-Resist component failures by stress corrosion cracking (SCC) in warm seawater. The failure has been attributed generally to castings that have not been stress-relief heat treated or in rare cases the cracking was associated with a high carbide contentin the iron. Miyasaka and Ogure3 have carried out a detailed investigation of thestress-corrosion cracking of austenitic irons studying the effect of applied stress, alloyingelements, temperature, NaCl concentrations, dissolved oxygen concentration andelectrode potentials. It has been shown that, at high stress levels, ductile type D2Ni-Resist would be susceptible to SCC in warm seawater. Failures have occurred incastings which were stress-relief heat treated. In such cases, local internal stresses closeto proof strength might occur (200 N/nm2) particularly if any weld repairs have beencarried out.

Dawson and Todd4 studied the influence of Carbide content on the stress corrosioncracking (SCC) of Ni-Resist cast irons in warm seawater (45%). It has been shownthat the exposure of highly stressed, Type D2 Ni-Resist castings to warm aeraetedseawater can promote the occurrence of SCC in fairly short time which could beminimized by a stress-relief heat treatment. They concluded that the form and amountof SCC likely to occur appears to be related to the carbide content of the iron, which isdetermined by the silicon and chromium contents and the efficiency of the inoculatingprocess used during production. Irons with high Si and relatively low Cr within thespecifications, which have been effectively inoculated to give the minimum amount ofcarbide in the structure, are least susceptible to SCC.

The studies on the behavior of Ni-Resist cast irons in seawater are motivated by a seriesof failures of Ni-Resist cast discharge columns of recirculation pumps of a seawaterdesalination plant in the Kingdom of Saudi Arabia. The studies are comprised of failureinvestigation on the discharge columns of brine recirculation pumps along with labo-ratory testings of Ni-Resist irons in seawater environments under different dissolvedoxygen concentrations using electrochemical methods of analysis and metallographicanalysis.

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2. STUDIES ON THE FAILED PUMP DISCHARGE COLUMN

2.1 General Observations

The failed brine recirculation pump discharge columns on inspection and physicalexamination showed that the columns were severely corroded and revealed deepmacro and micro cracks mostly in the upper column of pump (Fig.1). Microscopicobservations showed that the Ni-Resist casting used in the discharge columns washighly porous with patch welded areas. Thecasting was deleteriously corrodedand several macro cracks, some of them more than 1 mm wide and penetratingthe entire thickness of the discharge column, appeared mostly in the upper columnof the pump. Most of the cracks initiated from the inside surface of the dischargecolumn traversing longitudinally upwards culminating either within the wall sectionor at the upper surface. Interestingly, from these main cracks, many micro or semimicro cracks emanated. This was revealed by the microscopic examination of thecross-sections, and inner and outer surfaces of the failed discharge column spe-cimenss. A considerable number of the cracks emanated from the patch weldedzones although no crack was found on the welded area.

2.2 Results

2.2.1 Chemical Analvsis

Chemical analysis of a sample of the pump discharge material showed thatthe composition of the material was similar to that of Ni-Resist type D2(Table 1).

2.2.2. Microstructural Studies

Photomicrograph of a standard specimen of Ni-Resist D2 showed graphitenodules and lamellar carbides in an austenite matrix. The microstructure ofa specimen in the vicinity of weld/base metal interface showed progressionof cracks deep inside the section5. The cracks showed a tendency to passthrough the graphite nodules. Two types of structures were revealed. Thebase metal structure, which was porous showed the presence of one or twofine cracks originated from the weld/base metal interface and traversingsome of the graphite nodules. The structure of the weld zone revealed adendrite type structure in which Cr7C3 grains are entangled in betweenlamellar intermetallic phase5.

The microstructures of the specimen prepared by cutting through the crackedareas revealed the penetration of the crack into the section. These aretransgranular cracks from which several fine cracks emanated.

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The passage of cracks in the failed material was also studied through scanningelectron micrography - the cracks have progressed mainly through the aus-tenitic phase and show extensive branchings. The energy dispersive X-ray(EDAX) profiles of the specimen prepared by cutting through cracked areain the vicinity of weld zone show much higher concentration of Ni and Crin the weld metal than in the base metals. SEM pictures have also been takenfor cracks originated from the pores found on the surface of the casting nearthe weld zones. The cracks propagate through the austenitic matrix andculminate only at the deep cavities or pit holes found on the surface of thedischarge column.

2.3 Discussion

Observation of the pattern of cracking provide indisputable evidence that thefailure occurred predominantly by stress corrosion cracking (SCC). Frommetallographic studies it is evident that there are 2 possible sites from which thecracks have initiated. The pores in the pump discharge column casting have stressconcentrations and cracks could initiate from these pores progressing longitudi-nally from the inner surface of the discharge column to the outer surface or cul-minating in the midway of the wall section. In either case, fine cracks emanatedfrom the big cracks. The other region where cracks originated are in the vicinityof weld zone. These regions have greater concentration of stresses and the patternof cracking is similar to that observed where cracks initiate from pores albeit thereare fewer number of cracks.

In both cases, the cracks progress by passing through the graphite nodules andaustenitic phase or through austenitic phase only. It has been reported’ that SCCis most likely to occur more rapidly in areas that have been ground to smoothsurface for welding, the grinding process removes original casting skin.

The discharge column material castings have higher silicon and low chromiumcontents and therefore, the microstructure has predominant concentration ofgraphite and smaller carbide concentration which is apparently more resistant tocorrosion. However, in the region near the weld/base metal interface there is veryhigh chromium content resulting in an abnormal rise in carbide concentration.The region near the weld/base metal interface is the preferential site for the ini-tiation and propagation of SCC cracking. The cracks initiated from this area arefewer but are larger and penetrate more deeply into the section as is evident frommetallographic studies.

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3 . STUDIES ON THE CORROSION BEHAVIOR OF NI-RESIST IN SEAWATERENVIRONMENT

3.1 Experimental

The corrosion behavior of Ni-Resist alloys in seawater environment was studiedby the following techniques :

1. Electrochemical studies involving measurement of polarization resistanceand determination of Tafel plots.

2 . Measurement of free corrosion potentials with time.

3 . Weight loss measurements.

3.1.1. Techniques and General Procedure

The Electrochemical measurements were carried out under two variables :temperature and deaeration/aeration conditions. Four temperatures wereused : ambient (~25oC), 40oC, 60oC and 80oC. Three different aerationconditions were used : (1) Full deaeration by purging the seawater solutionwith purified nitrogen; the nitrogen from cylinders was bubbled through analkaline solution of pyrogallol to remove the traces of oxygen. (2) Partialdeaeration by purging the solution with nitrogen from cylinders directly. Thisnitrogen contains a few ppm of O 2. (3) Full aeration by bubbling air throughthe solution.

The weight loss measurements were conducted under conditions similar tothose used in electrochemical studies. The corrosion rates were determinedby following standard test procedure (ASTM,G31-72) by exposing couponsand measuring the weight loss due to corrosion.

Electrochemical polarization experiments were carried out on an EG&Gmodel 342-2 soft Corr measurement system. The system is consisted ofmodel 273 potentiostat/galvanostat, model 342 corrosion software and model30 IBM PS-2. All the experiments were carried out using a corrosion cellwith saturated calomel as reference electrode (SCE) and graphite as counterelectrode (EG&G model K 0047).

Open circuit corrosion potential (OCP) were recorded under fully deaerated,partially deaerated and fully aerated conditions. For these measurements,a separate cell with Ni-Resist coupon as WE and SCE as reference electrodewas used. It took 24-43 hrs to achieve a constant potential correspondingto OCP.

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Tafel plot measurements were carried out using a scan rate of 0.1 mV/Scommencing at a potential about 2.50 mV more active than the stable OCPand terminating at a potential 250 mV more positive than OCP. Beforestarting the polarization scan, the specimen in the sample holder (WE) wasleft in the cell for about 2 hr for attaining a steady state which is shown by anear constant potential and current at the commencement of the experiment.All potentials were measured vs SCE.

Polarization resistance measurements were conducted at a scan rate of 0.1mV/S with starting and final potentials corresponding to -20 mV and + 20mV Vs OCP, respectively. The max. current range was 10 uA.

3.1.2 Materials

Two types of Ni-Resist alloys, namely Ni-Resist D2 and Ni Resist T2 wereused for corrosion studies. The samples used for Ni-Resist D2 were obtained

from sound portion of the failed brine recirculation pump discharge column.After preparing the samples, they were examined carefully under microscopeto confirm that they were free from any defect, like cracks, porosities etc.Ni-Resist T2 coupons were obtained from Metal Samples, Munford, U.S.A.The chemical composition of the two Ni-Resist alloys used in the studies isgiven in Table 1.

Chemical composition of the artificial sea water used in the tests is given inTable 2. This water has a composition which is very similar to that of seawaterfrom Arbian Gulf.

3.2 Results

3.2.1 Metallographic Studies

Figure 1 shows microstructure of Ni-Resist D2 while Fig.2 shows the structureof Ni-Resist T2. The main difference between the two structures is in theforms of graphite-D2 has the graphite in the nodular form while in T2 it isin the form of flakes.

3.2.2. Electrochemical Studies

Polarization resistance (PR) curves obtained for Ni-Resist D2 at differenttemperatures under complete deaeration with purified nitrogen are shownin Fig.3. Figures 4 and 5 show the PR curves under partial deaeration andfull aeration conditions, respectively for Ni-Resist D2. Similar curves forNi-Resist T2 are shown in Figures 6 to 8. Figures 9 to 14 present Tafel plots

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for Ni-Resist samples under varying experimental conditions. Table 3presents the results of all electrochemical tests (PR and Tafel experiments)conducted on Ni Resist D2 at different temperatures under the threedeaeration conditions. Similarly, Table 4 presents the results of all electro-chemical tests conducted on Ni Resist T2.

3.2.3. Free Corrosion Potential Measurements

Plots of free corrosion potential vs time for Ni Resist D2 at different tem-peratures under fully deaerated conditions are shown in Fig.15 The plotsof free corrosion potential vs time for Ni Resist D2 under partially deaeratedcondition are given in Fig.16 while Fig.17 shows the free corrosion potentialvs time plots for Ni Resist D2 under fully aerated condition. The potentialtime plots for Ni Resist T2 under fully deaerated, partially deaerated andfully aerated conditions, respectively, are shown in Fig.18 to 20.

3.2.4 Weight Loss Experiments

Corrosion rates as determined from weight loss measurements for Ni ResistD2 alloy under conditions of full deaeration, partial deaeration and fullaeration at 40oC and 60°C are presented in Table 5.

3.3 Discussion

Some interesting features relating to the corrosion behavior of Ni-Resist alloys insea water are revealed by electrochemical studies.

As expected the corrosion rates of Ni-resist alloys in artificial seawater under fullyaerated condition are much higher (at least by one order of magnitude) than underfully deaerated or partially deaerated conditions. However, the corrosion ratesof Ni-resist alloys under fully deaerated or partially deaerated condition are onlymarginally different. This is presumably due to very small difference in oxygenactivities under the two conditions.

Considering the effect of temperature on corrosion rates, it is noted that in mostcases, the highest corrosion rate is found in the temperature range of 40 to 60oC.While explaining this behavior, two factors are to be considered: negative effectof temperature on dissolution of oxygen in water and positive effect of temperatureon the kinetics of metal dissolution or oxidation. The temperature range of40oC-60°C provides an optimum combination of two factors so as to exhibitmaximum corrosion rates.

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In any condition, the corrosion rates of Ni Resist T2 alloys are higher than NiResist D2 alloys. A graphite flake structure (exhibited by T2 alloys) is perhapsmore accessible to corrosion than a graphite nodular type structure exhibited byNi Resist D2. The corrosion rates obtained from the Tafel plots are invariablymuch lower(in some cases lower by a factor of 2 to 5) than those obtained fromthe respective polarization resistance measurements.

The simplest and most plausible explanation appears to be that in polarizationresistance measurements, the range of potential traverse is very small : within+20mV from the corrosion potential. Because of such a small potential range appliedon the surface, its surface characteristic would not have changed appreciably duringthe period of P.R. experiment. On the other hand, in Tafel measurements, thepotential traverse is much greater : +200 to 250 mV from the corrosion potential,so that during scan period the surface characteristic of the specimen changed insuch a way so as to resulting in lowering of corrosion rates.

The values of corrosion rates of Ni-resist irons as determined from weight lossmeasurements were far less than the values obtained from electrochemicalmethods under similar conditions of temperature and aeration or deaeration.Under either conditions of aeration, the corrosion rates obtained by the electro-chemical techniques are several times higher than the rates determined fromweight losses incurred during 30 to 40 days immersion tests. The long termimmersion of Ni-resist coupons in artificial seawater results in the formation ofthick and adherent scales of iron oxides which act as a barrier against furthercorrosion. As the corrosion rate has been calculated in terms of weight loss it isexpected that the corrosion rates could be reduced further on increasing theimmersion time.

Useful information is inferred from the study of free corrosion potential vs timeplots for Ni-resist D2 and T2 alloys under different experimental conditions. Boththe alloys behave almost similarly as far as the corrosion behavior is concerned.Under condition of complete deaeration, near passivity is achieved in a few hoursat room temperature. At other temperatures, the behavioral pattern of potentialvs time plots is rather irregular. Plateaus of constant potentials ranging from 6 to12 hrs are observed which are disturbed by a small drop or rise in potential. Ingeneral, regimes of steady potential are shown after about 50 hours. Initially, aprotective film is formed which is broken as indicated by a change in potentialfollowed by curing of the film which is indicated by a plateau and this process ofbreaking and curing of film goes on intermittently till a stable film is formed. Thevariations in potential are quite significant at 80oC where relatively less oxygen isavailable for oxide film formation.

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Under condition of partial deaeration, the corrosion behavior is similar to thatobserved in case of complete deaeration by purified nitrogen gas. The smallvariations in potential are observed intermittently. Each variation in potential isfollowed by a regime of constant potential represented by a plateau. Like previouscase alloy at 25oC acquires passivity within hours as shown by a constant potentialin the potential/time plot.

Near passivity is achieved within a few hours in Ni-resist under condition ofcomplete aeration. This behavior is observed at all the temperatures. Theinduction time appears to decrease linearly with increase in temperature (Fig.21).Under condition of complete aeration created by air bubbling, thick scales areformed which act as a barrier layer against further oxidation. Under partiallydeaerated or fully deaerated conditions, Ni-resist is not able to from protectivebarrier layer at temperatures above 25oC, because the oxide films formed areperhaps not sufficiently voluminous as well as adhered so as to envelope the wholealloy specimen.

An examination of the Tafel plot curves for the Ni-resist irons under differentaeration conditions indicates that only alloys under fully aerated conditions exhibitpassivity. The sign of passivity is indicated by a no change in current (at about l03

uA/cm2) with increasing potential. Under conditions of complete or partialdeaeration at no stage the Tafel plots show constancy or decrease in current withincreasing potential therefore, the possibility of attaining passivity is precluded.The indications of attainment of passivity under fully aerated conditions aredemonstrated by free corrosion potential vs time plots. At a particular temper-ature, the corrosion potential, Ecorr increases with increase in oxygen content butat a constant oxygen concentration, it decreases with increasing temperature.

A state of achievement of near passivity on Ni-Resist irons in seawater underdeaerated conditions is demonstrated by electrochemical measurements. How-ever, this might not represent a true passivity since the corrosion rates are invariablyhigher under decreasing dearation conditions as indicated by electrochemical andimmersion tests. In Ni-Resist irons a state of complete passivity will perhaps neverbe achieved because of the poor mechanical adherence of the oxide scales andonly a short term or temporary passivity would be observed as long as voluminousoxide scales are adhered to the metal.

4 CONCLUSION

1. Failure in the brine recirculation discharge column has occurred predominantly by stress corrosion cracking.

2. Cracks are initiated from the pores and progress by passing through the graphitenodules and austenitic phase or through austenitic phase only.

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3 .

4.

5 .

6 .

7 .

8 .

The cracking pattern is characterized by the emanation of fine cracks from the bigcracks.

The failed region near the weld/base metal interface which is the preferential sitefor SCC is characterized by high chromium content resulting in an abnormal risein carbide concentration.

Ni-Resist irons with graphite flake structure have higher corrosion rates than thecorresponding alloys with graphite nodule structure.

Maximum corrosion rates for Ni-Resist irons are exhibited in the temperaturerange of 40 - 60oC.

The corrosion rates for Ni-Resist irons decrease with decreasing dissolved oxygenconcentration being lowest under deaerated conditions. However, at no stage apassive region in the electro-chemical curves is observed.

Only under conditions of full deaeration, limited passivity is observed. The lackof passivity is attributed to the weak adhesion of the iron oxide scales on the alloy.

REFERENCES

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2 .

3 .

4 .

5 .

T.P.May, J.F.Mason, Jr., and W.K.Abbot,Materials Protection, 1, 40(1961).B.D.Criag and L.A. McClymondsCorrosion, 37,485 (1981).M.Miyasaka and N.Ogure,Stress corrosion cracking of austenitic cast irons in seawater and brine and itsprevention. Corrosion 86, Paper No. 324, Houston, Texas.J.V. Dowson and B. Todd,Influence of carbide content on the stress corrosion cracking of Ni-Resist cast ironsin warm seawater BCIRA Journal, November 1987 (Ni DI Technical Series No.10018 pp 1-9.A.U.Malik and Shahreer AhmedA study on the discharge columns of brine recycle pumps in Phase II desalinationplant at Al-Jubail.Research Activities and Studies, Volume 1 - 1410H (1990)

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corrosion behaviour if ni-resist cast irons in seawater

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