evaluations of long-term seawater exposure corrosion … · 2012. 8. 3. · 13 mois était de 0,52...

Defence R&D Canada – Atlantic

DEFENCE DÉFENSE&

Evaluations of Long-Term Seawater

Exposure Corrosion Specimens and

Electrochemical Studies of Nickel

Aluminum Bronze Alloys

Yueping Wang

Technical Memorandum

DRDC Atlantic TM 2008-256

December 2008

Copy No. _____

Defence Research andDevelopment Canada

Recherche et développementpour la défense Canada

This page intentionally left blank.

Evaluations of Long-Term Seawater Exposure Corrosion Specimens and Electrochemical Studies of Nickel Aluminum Bronze Alloys

Yueping Wang

Defence R&D Canada – Atlantic Technical Memorandum DRDC Atlantic TM 2008-256 December 2008

Principal Author

Yueping Wang

Defence Scientist

Approved by

Leon Cheng

Head/DLA

Approved for release by

Calvin Hyatt

DRP Chair

© Her Majesty the Queen in Right of Canada, as represented by the Minister of National Defence, 2008

© Sa Majesté la Reine (en droit du Canada), telle que représentée par le ministre de la Défense nationale, 2008

Original signed by Yueping Wang

Original signed by Leon Cheng

Original signed by Ron Kuwahara for

Abstract ……..

Cast nickel aluminum bronzes (NAB) are widely used in submarine seawater handling systems. However, they are susceptible to corrosion, especially selective phase corrosion (SPC), at welds and under marine growth. This leads to high repair or replacement costs. A first step in reducing these costs is to understand the effect of local environmental factors. In order to assess local environmental factors affecting the corrosion performance of NAB alloys, a long-term seawater exposure experiment was conducted. Four sets of NAB castings, including welded specimens, were immersed in natural seawater in Bedford Basin, Nova Scotia for 7, 13 and 39 months. Corrosion performance was evaluated through visual inspection, weight loss and cross-section metallographic examination. In addition, electrochemical techniques were used to study the effect of decaying biological matter on the corrosion behaviour of NAB castings.

This technical memorandum presents the analysis results of the long-term exposure experiments including general corrosion and SPC in the weld heat-affected zone and under crevices formed by nylon nuts and bolts, along with local environmental conditions (dissolved oxygen, temperature, and salinity). The electrochemical study on the effect of decaying biological matter on the corrosion behaviour of NAB alloys is also presented and discussed.

Résumé ….....

Les pièces moulées en bronze au nickel-aluminium sont largement utilisées dans les systèmes de traitement d’eau de mer des sous-marins. Cependant, elles sont sujettes à la corrosion, en particulier la corrosion sélective (CS), au niveau des soudures et sous les salissures marines. Ce phénomène se traduit par des coûts de réparation ou de remplacement élevés. La première étape pour réduire ces coûts consiste à comprendre l’effet des facteurs environnementaux au niveau local. Afin d’évaluer ces facteurs qui influent sur la résistance à la corrosion des alliages de bronze au nickel-aluminium, une expérience d’exposition à long terme à l’eau de mer a été menée. Quatre ensembles de pièces moulées en bronze au nickel-aluminium, y compris des pièces soudées, ont été plongés dans l’eau de mer naturelle dans le bassin de Bedford, en Nouvelle-Écosse, pendant des périodes respectives de 7, 13 et 39 mois. La résistance à la corrosion a été évaluée au moyen d’une inspection visuelle, de la mesure de la perte de poids et d’un examen métallographique sur le plan transversal. De plus, des techniques électrochimiques ont servi à l’étude de l’effet des matières biologiques en décomposition sur la résistance à la corrosion des pièces moulées en bronze au nickel-aluminium.

Le présent mémorandum technique présente les résultats de l’analyse des expériences d’exposition à long terme, y compris la corrosion générale, et la corrosion SC au niveau des endroits soudés affectés thermiquement et sous les lézardes formées par des écrous et des boulons en nylon, de même que les conditions environnementales locales (oxygène en solution, température et salinité). L’étude électrochimique de l’effet des matières biologiques en décomposition sur la résistance à la corrosion des alliages en bronze au nickel-aluminium est étalement présentée et traitée.

DRDC Atlantic TM 2008-256 i


ii DRDC Atlantic TM 2008-256

Executive summary


Yueping Wang; DRDC Atlantic TM 2008-256; Defence R&D Canada – Atlantic; December 2008.

Introduction: Cast nickel aluminum bronzes (NAB) are widely used in submarine seawater handling systems. However, they are susceptible to corrosion, especially selective phase corrosion (SPC), at welds and under marine growth. This leads to high repair or replacement costs. In order to assess local environmental factors affecting the corrosion performance of NAB alloys, a long-term seawater exposure experiment was conducted. Four sets of NAB castings, including welded specimens, were immersed in natural seawater in Bedford Basin, Nova Scotia for 7, 13 and 39 months. Corrosion performance was evaluated through visual inspection, weight loss and cross-section metallographic examination. In addition, electrochemical techniques were used to study the effect of decaying biological matter on the corrosion behaviour of NAB castings.

Results:

1. No appreciable corrosion was observed in the NAB specimens after 7 months of immersion in Bedford Basin. 2. SPC occurred in all 13-month and the remaining 39-month NAB specimens. SPC was found to have occurred inside the crevices, in the areas adjoining the crevices formed by the nylon washers, and along the heat-affected zone (HAZ) in the welded specimens. SPC was also found on the bare surfaces where debris of decaying marine organisms, sulphide-containing black films, and deposit of corrosion products acted as crevice formers. 3. The maximum corrosion rate for the 13-month specimens was found to be 0.52 mm/year. The corrosion rate for a single specimen was 0.29 mm/year over the 39 months of seawater immersion. The maximum corrosion rate along the weld HAZ was found to be 2 mm/year, which is the highest corrosion rate reported in literature. 4. The short-term (2-week) electrochemical study results showed that the decaying marine organisms resulted in a -400 mV shift in the corrosion potentials, and decreased corrosion rate, due to consumption of the oxygen in the seawater.

Significance: The current study is part of the effort in understanding environmental factors affecting corrosion performance of NAB castings used in submarine seawater handling systems. The experimental results can be used to help improve fitness-for-service assessment of NAB components in the VICTORIA Class submarine seawater handling systems.

Future plans: Long-term (at least 3 months) electrochemical studies are to be conducted to quantify the long-term effect of decaying marine organisms on the corrosion performance of NAB specimens.

DRDC Atlantic TM 2008-256 iii

Sommaire .....


Yueping Wang; DRDC Atlantic TM 2008-256; R et D pour la défense Canada – Atlantic; décembre 2008

Introduction : Les pièces moulées en bronze au nickel-aluminium sont largement utilisées dans les systèmes de traitement d’eau de mer des sous-marins. Cependant, elles sont sujettes à la corrosion, en particulier la corrosion sélective (CS), au niveau des soudures et sous les salissures marines. Ce phénomène se traduit par des coûts de réparation ou de remplacement élevés. Afin d’évaluer ces facteurs qui influent sur la résistance à la corrosion des alliages de bronze au nickel-aluminium, une expérience d’exposition à long terme à l’eau de mer a été menée. Quatre ensembles de pièces moulées en bronze au nickel-aluminium, y compris des pièces soudées, ont été plongés dans l’eau de mer naturelle du bassin de Bedford, en Nouvelle-Écosse, pendant des périodes respectives de 7, 13 et 39 mois. La résistance à la corrosion a été évaluée au moyen d’une inspection visuelle, de la mesure de la perte de poids et d’un examen métallographique sur le plan transversal. De plus, des techniques électrochimiques ont servi à l’étude de l’effet des matières biologiques en décomposition sur la résistance à la corrosion des pièces moulées en bronze au nickel-aluminium.

Résultats :

1. On n’a observé aucune corrosion notable sur les échantillons de bronze au nickel-aluminium au bout de 7 mois d’immersion dans le bassin de Bedford.

2. La corrosion CS s’est produite dans tous les échantillons immergés pendant 13 mois et les autres, qui ont été immergés pendant 39 mois. On a constaté que la CS se produisait à l’intérieur des lézardes, aux endroits adjacents à ces dernières, formées par des bagues en nylon, et le long des zones affectées thermiquement sur les échantillons soudés. On a également décelé de la CS sur des surfaces nues où des débris d’organismes marins en décomposition, des films noirs contenant des sulphides et des dépôts de produits de la corrosion agissaient en tant qu’agents formant des lézardes. 3. On a constaté que le taux de corrosion maximal pour les échantillons immergés pendant 13 mois était de 0,52 mm/an. Le taux de corrosion pour un seul échantillon était de 0,29 mm/an pendant la période de 39 mois d’immersion dans l’eau de mer. Le taux de corrosion maximal le long des zones soudées affectées thermiquement était de 2 mm/an, soit le taux de corrosion le plus élevé rapporté dans la documentation. 4. Les résultats de l’étude électrochimique à court terme (2 semaines) ont démontré que les organismes marins en décomposition entraînaient une variation de -400 mV dans le potentiel de corrosion et diminuaient le taux de corrosion, en raison de la consommation de l’oxygène présent dans l’eau de mer.

Portée : L’étude courante fait partie de l’effort pour comprendre les facteurs environnementaux qui influent sur la résistance à la corrosion des pièces moulées en bronze au nickel-aluminium des

iv DRDC Atlantic TM 2008-256

systèmes de traitement d’eau de mer des sous-marins. Les résultats expérimentaux peuvent contribuer à améliorer l’évaluation de l’aptitude fonctionnelle des éléments en bronze au nickel-aluminium des systèmes de traitement d’eau de mer à bord des sous-marins de la classe VICTORIA.

Recherches futures : Des études électrochimiques à long terme (au moins 3 mois) doivent être menées pour quantifier l’effet à long terme des organismes marins en décomposition sur la résistance à la corrosion des échantillons en bronze au nickel-aluminium.

DRDC Atlantic TM 2008-256 v


vi DRDC Atlantic TM 2008-256

Table of contents

................................................................................................................................. i Abstract ……..................................................................................................................................... i Résumé ….....

........................................................................................................................ iii Executive summary.................................................................................................................................. iv Sommaire .....

........................................................................................................................... vii Table of contents............................................................................................................................... viii List of figures

.................................................................................................................................... x List of tables........................................................................................................................ xi Acknowledgements

............................................................................................................................... 1 1 Introduction ............................................................................................................ 3 2 Experimental Procedure

2.1 .................................................................................................... 3 Specimen Preparation2.2 ........................................................................................... 3 Seawater Exposure History2.3 ........................................................................................................ 5 Specimen Cleaning2.4 .............................................................................. 5 Metallographic Sample Preparation2.5 ................................................................................................ 5 Electrochemical Testing

.............................................................................................................. 7 3 Resuls and Discussion3.1 ................................................................................... 7 Bedford Basin Water Conditions3.2 ....................................................................................................... 7 Visual Examination

3.2.1 ................................................................................. 7 Seven-month specimens.3.2.2 ............................................................................. 9 Thirteen-month specimens.3.2.3 ...................................................................... 12 Thirty-nine-month specimens.

3.3 .................................................................................. 13 EDX Analysis of Black Deposit3.4 ....................................................................................... 16 Metallographic Examination

3.4.1 ............................................................................................... 16 Microstructure3.4.2 ................................................................................. 20 Corrosion in weld HAZ3.4.3 ............................................................................... 20 Corrosion under crevices3.4.4 ............................................................................ 27 Corrosion on bare surfaces

3.5 ........................................................................................................... 28 Corrosion Rates3.6 ................................................................................. 30 Electrochemical Testing Results3.7 ................................................ 32 Discussion on Effects of Decaying Biological Matter

..................................................................................................... 34 4 Summary and Conclusions ................................................................................................................... 35 5 Recommendations ............................................................................................................................... 36 6 References

.............................................................................................................................. 37 Distribution list

DRDC Atlantic TM 2008-256 vii

List of figures

Figure 1: Coupons arrangement before the exposure (a) and the way each coupon was attached to the PVC rack (b). ........................................................................................ 4

Figure 2: The setup for the electrochemical testing: (a) two loops of four cells each and (b) a close-up view of an individual cell (WE – working electrode (specimen); RE – reference electrode; CE – counter electrode). ............................................................... 6

Figure 3: Water temperature, dissolved oxygen level and salinity in Bedford Basin during the NAB exposure testing [12]............................................................................................ 8

Figure 4: Surface appearance of coupons A7, B7, C7 and 4A after 7 months of seawater exposure. ....................................................................................................................... 9

Figure 5: Specimens after 7 months of exposure in Bedford Basin followed with 3 months of exposure in the indoor seawater tank. ......................................................................... 10

Figure 6: Surface appearance of Coupons A8, B9, C8 and 5A after 13-month seawater exposure. ..................................................................................................................... 11

Figure 7: Surface appearance of 13 month exposed Coupons A8, B9, C8 and 5A after chemical cleaning........................................................................................................ 12

Figure 8: Specimens after 7 months of exposure in Bedford Basin followed with 3 months of exposure in the indoor seawater tank. ......................................................................... 13

................... 14 Figure 9: Surface appearance of Coupon 4B after 39 months of seawater exposure.

Figure 10: Surface appearance of 39 month exposed Coupon 4B after chemical cleaning, showing localized corrosion and re-deposited copper in and around the corroded areas............................................................................................................................. 15

.................... 16 Figure 11: The energy dispersive X-ray spectrum of the black deposit on the rack.

............................................................ 17 Figure 12: Microstructure of Specimen 4A, parent metal.

.................................................................................. 17 Figure 13: Microstructure of Specimen A8.

.................................................................................. 18 Figure 14: Microstructure of Specimen B9.

.................................................................................. 19 Figure 15: Microstructure of Specimen C8.

Figure 16: Cross section micrograph showing (a) weld/parent metal interface for specimen 4A, with inset showing pre-etching cross section in a selected area; and (b) microstructure across the HAZ (after 7 months of immersion). ................................. 21

Figure 17: Cross section micrograph showing the weld/parent metal interface for specimen 5A (after 13 months of immersion)............................................................................. 22

Figure 18: Cross section micrograph near the front surface showing re-deposited copper in the areas where selective phase corrosion took place (Specimen 5A, after 13 months of immersion). ................................................................................................ 22

Figure 19: Cross section micrograph showing selective phase corrosion near the weld fusion line at the front surface for specimen 4B (after 39 months of immersion). ................ 23

viii DRDC Atlantic TM 2008-256

Figure 20: Cross section micrograph showing selective phase corrosion and re-deposited copper in the corroded areas in the heat-affected zone 4 mm from the front surface of specimen 4B (after 39 months of immersion)......................................................... 23

Figure 21: Cross section micrograph showing selective phase corrosion and re-deposited copper in the corroded areas in the heat-affected zone 6 mm from the front surface of specimen 4B (after 39 months of immersion)......................................................... 24

Figure 22: Cross section micrograph showing crevice corrosion in Specimen A8 (after 13 months of immersion). ................................................................................................ 24

Figure 23: Cross section micrograph showing crevice corrosion at front surface in Specimen 4B (after 39 months of immersion). ............................................................................ 25

Figure 24: Cross section sample and micrograph showing crevice corrosion at front surface adjoining the mouth of a crevice formed by the nylon washer (Specimen B9, after 13 months of immersion). ........................................................................................... 25

Figure 25: Photograph showing location of corrosion area and cross section micrograph showing crevice corrosion at front surface near the crevice formed by the nylon washer (Specimen 5A, after 13 months of immersion)............................................... 26

Figure 26: Cross section micrograph showing selective phase corrosion at one end surface in Specimen C8 (after 13 months of immersion). ........................................................... 27

Figure 27: Micrograph showing preferential attack on martensitic β phase resulting in the loss of grains (Specimen C8, after 13 months of immersion). ........................................... 28

Figure 28: Cross section micrograph showing selective phase corrosion at one end surface (i.e. Figure 10c) in Specimen 4B (after 39 months immersion).................................. 29

Figure 29: Maximum corrosion rates of NAB specimens after 7, 13, and 39 months of seawater immersion..................................................................................................... 30

Figure 30: Corrosion potential (EOC) as a function of immersion time (Specimens A8, B9, C8 and 5A in Loop one; Specimens A8′, B9′, C8′ and 5A′ in Loop two with PVC rack submerged in the seawater tank after three-day testing). ............................................ 31

Figure 31: Corrosion rate (1/RP) as a function of immersion time (Specimens A8, B9, C8 and 5A in Loop one; Specimen A8′, B9′, C8′ and 5A′ in Loop two with PVC rack submerged in the seawater tank after three-day testing). ............................................ 32

DRDC Atlantic TM 2008-256 ix

List of tables

.................................................................. 3 Table 1: Chemical Composition (wt.%) of Cast Plates.

x DRDC Atlantic TM 2008-256

Acknowledgements

The author would like to acknowledge Dr. Calvin Hyatt for initiating this project and for organizing the NAB samples used in seawater exposure testing, and Ms. Donna Reimchen for assisting in the electrochemical experiments and metallographic examination throughout her summer work term.

DRDC Atlantic TM 2008-256 xi


xii DRDC Atlantic TM 2008-256

1 Introduction

Cast nickel aluminum bronzes (NAB) are widely used in submarine seawater handling systems. The alloys were introduced to the submarine seawater systems in mid-1960s because of their superior strength and shock resistance, and reasonably good corrosion resistance [1]. However, NAB castings were found to be susceptible to selective phase corrosion (SPC), at welds, at crevices and under marine growth. This has led to high repair or replacement costs.

NAB is a complex, multi-phase copper-based alloy that contains a number of inter-metallic phases. Some of these phases are more anodic than others, rendering them especially vulnerable to SPC [2]. NAB castings with a composition of 10% Al, 5% Ni, and 5% Fe generally contain the following phases: a copper-rich α phase, iron-rich κ and κ phases, a nickel-rich κI II III phase, and an iron-rich κIV phase [2]. In addition, depending on the cooling rate, as-cast NAB also contains an aluminum-rich martensitic β phase. The martensitic β phase is also present in the heat-affected zone (HAZ) of welds as a result of the reconstitution of the high temperature β phase from the α phase and various κ phases, and transformation of the high temperature β phase to martensitic β phase on subsequent rapid cooling.

Early research focused on the study of the mechanism of SPC of the NAB castings [3-5]. Local pH on the NAB castings appeared to have a major effect on the mechanism of SPC. Rowlands [5] observed through crevice corrosion experiments that the copper-rich α phase was initially anodic to Al-Fe-Ni rich κIII phase and corroded preferentially at a low rate in slightly alkaline seawater (pH 8.2). In the meantime, the buildup of hydrogen ion in the crevice gradually transformed the seawater in the crevice from pH 8.2 to pH 3. At this pH level, the κIII phase had become anodic to the α phase and was preferentially corroding at the rate of 0.7-1.1 mm/year. The reversal of the galvanic effect took about 5 months. In the presence of martensitic β phase, the β phase was anodic to the α phase and preferentially corroded [3].

The preferential corrosion of the martensitic β phase also took place in the HAZ of welded samples. The attack was most severe in the parts of HAZ where the areas of the martensitic β phase were smallest, possibly due to the small area of the anodic β phase in comparison to the large area of cathodic κ and α phases [3].

Destructive examinations carried out by Galthworthy et al. [1] on over 100 NAB castings after 7 to 13.5 years of service in seawater handling systems have shown the variability in extent and depth of SPC in these components. They observed through-wall SPC on some platform specific valves, although the corrosion rates have not appeared to exceed 1.1 mm/year; the maximum corrosion rate quoted for this alloy. DRDC Atlantic destructively examined a number of NAB castings and carried out revalidation on 25 NAB components after 15 years of service [6]. Both layer SPC and plug SPC were observed. The corrosion appeared to be less severe than that observed in the Royal Navy submarine NAB castings.

It has been suggested that local seawater environments could be important factors affecting the corrosion performance of NAB castings, although little work had been done on local environmental effects in the early years. Local seawater environments (temperature, oxygen level and salinity) affect the pattern and rate of marine biological growth, which in turn determine the

DRDC Atlantic TM 2008-256 1

characteristics of crevices formed by marine growth and decaying marine biological matter. The effects of local seawater environments on NAB corrosion have been the subject of recent research [7-8].

In order to assess local environmental factors affecting the corrosion performance of NAB alloys, long-term seawater exposure experiments were conducted. Four sets of NAB castings, including gas metal arc welded (GMAW) specimens, were immersed in seawater in Bedford Basin, Nova Scotia for 7, 13 and 39 months. Corrosion performance was evaluated through visual inspection, weight loss and cross-section metallographic examination. In addition, the short-term effect of decaying marine biological matter on the corrosion behaviour of NAB castings was studied using electrochemical techniques.

This study was carried out as part of The Technical Cooperation Program (TTCP) Operating Assignment on environmental factors affecting the corrosion performance of nickel aluminum bronze (MAT-TP1-OA36), under which long-term exposure experiments using the same batches of NAB coupons supplied by Defence R&D Canada – Atlantic were also conducted in the UK and Australia. The Operating Assignment was completed in 2007 with a final report summarizing the major results obtained from all three participating countries [9]. This technical memorandum presents detailed analysis results of the long-term exposure experiments that were conducted at Defence R&D Canada – Atlantic.

2 DRDC Atlantic TM 2008-256

2 Experimental Procedure

2.1 Specimen Preparation

The samples, collected from four different sources (three samples per source), were machined to 50 mm long, 25 mm wide and 25 mm thick, except for the welded cast which was machined to 76 mm long, 25 mm wide, and 20 mm thick. The sources and the chemical composition of the specimens as well as the stamp numbers are presented in Table 1.

Table 1: Chemical Composition (wt.%) of Cast Plates.

Sample Stamped Al % Ni% Fe% Mn% Pb% Zn% Cu% Scrap Propeller (DRDC analysis)

9.08 4.71 4.53 1.58 Nil 0.02 rem A7, A8, A9

Scrap Propeller (Bodycote Technitrol analysis)

9.36 4.64 4.44 1.43 0.005 nil 79.70

Valve Gate Plate (NES 747 Pt2)

B7, B8, B9 8.8 4.74 4.38 1.25 Nil nil rem

Sand Cast* C7, C8, C9 8.9 4.7 4.1 1.2 Nil nil rem Welded Cast (Parent Material)

4A, 4B, 5A 9.1 4.70 4.53 1.58 Nil 0.03 rem

Welded Cast (wire part)

4A, 4B, 5A 9.0 4.15 3.81 1.17 Nil nil rem

*provided by CANMET – Material Technology Laboratory, Natural Resources Canada

A PVC rack, 610 mm square and 6.4 mm thick, was used to hold all 12 coupons (Figure 1a). Each coupon was hand-tightened using nylon nuts, bolts, and washers before being fastened to the PVC plate (Figure 1b). The gap between the coupons and the PVC plate was 8.7 mm. Figure 1b was also annotated to define different surfaces on a NAB coupon. For “A”, “B” and “C” coupons, the front surfaces were the ones on which the identifications of the coupons were stamped. In the case of welded coupons, the front surfaces were the surfaces with weld face reinforcement.

2.2 Seawater Exposure History

The seawater exposure test started in March 2003. The specimens were exposed to seawater by hanging the PVC rack to the Defence R&D Canada – Atlantic acoustic calibration barge in Bedford Basin, Halifax, Nova Scotia. The rack was placed 10 m below the water surface in the first 7 months and 2 m below water surface in the remaining exposure period. A weight, as shown in Figure 1a, was attached to the rack in order to maintain the rack in a vertical position throughout the exposure period. The three batches of specimens were retrieved from the rack after 7 months, 13 months and 39 months of exposure. Due to a requirement to repair hurricane damage to the barge during the testing period, the PVC rack was removed from Bedford Basin and vertically placed in an indoor tank filled with unfiltered static seawater between the 8th month and 11th month and between the 17th month and 20th month. The altered exposure procedure


provided an opportunity to examine the effect of decaying microbiological matter on the corrosion performance of the NAB specimens.

C9 C7 C8 B8 B7 B9

A9 A7 A8 4B 4A 5A

Weight

(a)

End

BackFront

Side

(b)

Figure 1: Coupons arrangement before the exposure (a) and the way each coupon was attached to the PVC rack (b).


2.3 Specimen Cleaning

Loose corrosion products were gently brushed from the surface of the specimens. A solution of 10% sulphuric acid was used to clean the specimens for 3 to 5 minutes. For the 13-month and 39-month specimens, a solution of 50% hydrochloric acid was also used to remove the corrosion products. During the chemical cleaning, brushes were used to help remove the corrosion products. For the 39-month coupon, a glass rod was also used to scrape off the corrosion product during the chemical cleaning. Care was taken not to cause any damage to the coupon surfaces.

2.4 Metallographic Sample Preparation

Selected specimens were sectioned and mounted, exposing areas of interest for metallographic examination. The welded specimens were sectioned in a manner that showed the weld/parent metal interfaces from a side view. For both welded and un-welded specimens, locations with the most severe surface pitting were chosen for cross section metallographic examination.

The samples were cut using an abrasive saw and mounted in a phenolic with EPOMET®1 added to enhance edge retention. The specimens were ground progressively using SiC paper of up to 600 grit, and polished with diamond slurries of 6 μm to 0.25 μm. The specimens were then etched in 10% FeCl3 solution for 10 to 15 seconds, rinsed and air dried.

2.5 Electrochemical Testing

Electrochemical testing was conducted to investigate the effect of decaying biological matter on the corrosion behaviour of the NAB specimens. Four 13-month coupons (i.e. A8, B9, C8 and parent metal of 5A) were cut using an abrasive saw to make smaller specimens for the electrochemical testing with the fresh saw-cut surfaces as exposed surfaces. Electric connection to each specimen was made by soldering a wire to the specimen. Each specimen was then mounted in an epoxy cylinder and progressively ground to 600 grit. The exposed surface areas of the specimens ranged from 1.2 cm2 to 2.5 cm2.

Two parallel loops were set up with 4 corrosion cells in each (Figure 2). Each loop had a set of four samples taken from each of the 13-month specimens and drew Halifax Harbour water from a separate tank. An aquarium pump and a siphoning unit were used in each loop to circulate the water between the cells and the tank. The flow rate in each loop was held constant at 0.8 L/min. After the testing ran for three days, the PVC rack and attached rope with residual biological matter was added to one of the seawater tanks.

1 An epoxy-based resin containing a hard filler material.


RE WE

CE

(a) (b)

Figure 2: The setup for the electrochemical testing: (a) two loops of four cells each and (b) a close-up view of an individual cell (WE – working electrode (specimen); RE – reference

electrode; CE – counter electrode).

The tank temperatures varied from 21.0°C to 22.5°C, while the pH of both tanks ranged from 6.0 to 6.5. Throughout the experiment, there was no significant difference between the two tanks with regard to the temperature or pH. The Gamry PC4/300 Corrosion Test System was used to conduct the electrochemical testing, including open circuit potential and linear polarization resistance measurements. The linear polarization resistance technique has been used in other studies to measure the corrosion rates of NAB alloys and other copper alloys in seawater [10, 11].


3 Resuls and Discussion

3.1 Bedford Basin Water Conditions

Bedford Basin has a surface area of 17 km2 and is connected to the Atlantic Ocean by a channel that is about 10 km long and only 400 m wide at its narrowest part. Discharge from the Sackville River and runoff from the surrounding areas provide an influx of fresh water into the basin.

The water conditions (i.e. temperature, salinity, dissolved oxygen) in Bedford Basin at 2 meters below water surface from January 2003 to June 2006 are presented in Figure 3 [12]. During the three years, the lowest water temperatures (i.e. from –0.8°C to –0.1°C) were obtained in late February or early March, and the highest water temperatures (from 17.2°C to 18.8°C) were obtained between July and September. The salinity in Bedford Basin was measured between 25.1 and 30.9 psu (practical salinity unit) during the three years. The average salinity in Bedford Basin (29.4 psu) was lower than that in open sea. The dissolved oxygen varied between 3.1 and 10.7 mL/L with the higher concentrations being recorded during the winter and spring, when the temperatures were lowest.

Data [12] also showed that on average the water temperature at 10 m below water surface was 3.2°C lower than that at 2 m. On the other hand, the salinity at 10 m was 1 psu higher and less variable than that at 2 m. The oxygen level was slightly lower (0.7 mL/L lower) at 10 m than at

m2 .

3.2 Visual Examination

3.2.1 Seven-month specimens.

There was a significant amount of biological growth, including mussels, sea anemone and barnacles, on the ropes and PVC rack when the PVC rack was pulled out of water after 7 months of immersion. There was also a small amount of biological matter attached to the nylon bolts; however, there was no biological growth directly attached to the NAB coupons.

Photographs, taken prior to cleaning, of specimens A7, B7, C7, and 4A are presented in Figure 4. A reasonably protective light brown film was observed on all NAB specimens. The only exception was the weld material in coupon 4A, which was covered by a black film. Localized corrosion was not observed on any of the specimens. However, a blue deposit was observed on coupon B7.

Chemical cleaning using 10% sulphuric acid did not reveal any localized corrosion. One visible change after the chemical cleaning was that the weld material had lightened in color from black to brown.


-5.0

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5

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ec-0

5

14-M

ar-0

6

12-J

un-0

6

10-S

ep-0

6

Date

Salin

ity (p

su)

Salinity (psu)

Figure 3: Water temperature, dissolved oxygen level and salinity in Bedford Basin during the NAB exposure testing [12].


A7 Front B7 Front

C7 Front 4A Front

4A Side

Figure 4: Surface appearance of coupons A7, B7, C7 and 4A after 7 months of seawater exposure.

3.2.2 Thirteen-month specimens.

The 13-month specimens were exposed in Bedford Basin for 7 months, immersed in the indoor seawater tank for 3 months, and then returned to the Bedford Basin for an additional 3 months. Before the specimens were placed in the indoor tank, most marine growth attached to the PVC rack and the ropes was scraped off; however, there was still residual marine growth attached to the rack and the rope. In the tank, the rack was maintained in the same vertical orientation as that when exposed in Bedford Basin. After a few days of immersion in the indoor tank, the tank water turned black and there was black deposit on the rack, rope and the NAB specimens. The tank was


flushed a few times in order to replace the black water with fresh seawater. After 3 months in the indoor stagnant tank seawater, a significant amount of corrosion deposit accumulated on the specimens, Figure 5. The tank seawater likely became anaerobic and resulted in the death of the residual marine organisms on the rack and ropes. Subsequent decaying of the dead organisms likely caused the discoloration of the stagnant tank seawater. The decaying of the organisms could have possibly accelerated the corrosion of the specimens.

Photographs of the second batch of specimens (i.e. A8, B9, C8 and 5A), taken before cleaning, are presented in Figure 6. It can be seen that a significant amount of multi-colored corrosion product formed on most of the specimens. After 13 months, the corrosion products were up to 4 mm thick. The black films on the specimens were likely the residual black deposit produced when the specimens were exposed in the stagnant seawater tank. Some black films were covered with light yellow and blue corrosion products.

Figure 7 shows the surface appearance of the four specimens after chemical cleaning. Crevice corrosion was observed in all specimens under the nylon washers and around the rims of the washers. Localized corrosion regions, covered with pink copper deposit, were also visible on both smooth surfaces and on the surfaces with machined grooves. For the welded specimen (5A), corrosion was observed along the fusion lines of the weld, in particular on the side surfaces. Localized corrosion with copper deposit was also visible on the weld material.

C9 C8 B8 B9

A9 A8 4B 4A

Figure 5: Specimens after 7 months of exposure in Bedford Basin followed with 3 months of exposure in the indoor seawater tank.


A8 Front B9 Front

C8 Front 5A Front

5A Side

Figure 6: Surface appearance of Coupons A8, B9, C8 and 5A after 13-month seawater exposure.


A8 Front B9 Front

C8 Front 5A Front

5A Side

Figure 7: Surface appearance of 13 month exposed Coupons A8, B9, C8 and 5A after chemical cleaning.

3.2.3 Thirty-nine-month specimens. rdThe 3 batch of specimens was transferred into the indoor seawater tank again at the 17th month

(September 2004) and moved back to Bedford Basin at the 20th month (December 2004). The second indoor exposure resulted in more corrosion product and black deposit on the specimens, Figure 8. The rack was retrieved for the final time on July 10, 2006. The rack had been damaged at some point and only the welded specimen 4B was recovered. As a result, the analysis of the NAB’s condition after 39 months of immersion was confined to work on specimen 4B.


C9 B8

A9 4B

(a) 4 NAB coupons on the rack (b) Close-up view of Coupon 4B

Figure 8: Specimens after 7 months of exposure in Bedford Basin followed with 3 months of exposure in the indoor seawater tank.

As shown in Figure 9, the specimen was covered with multi-colored corrosion product, as thick as 8 mm. The corrosion products were concentrated around the parent/weld interface on the front, back and both side surfaces.

Removal of the corrosion product using 50% HCl solution revealed extensive localized corrosion, characterized by copper deposit, on all surfaces of the specimen, including the weld material, Figure 10. In addition, crevice corrosion was observed under and around the nylon washers, as well as in the weld HAZ. The pattern of the macro-fouling on the specimen surfaces and the pattern of the localized corrosion on the end surfaces of the specimen, as shown in Figure 9 and Figure 10 suggest that the localized corrosion initiated under or around the crevices formed by the macro-fouling, including decayed biological matter.

3.3 EDX Analysis of Black Deposit

Figure 8a showed a photograph of the last batch of four specimens that was taken one month after the rack was moved to the indoor seawater tank for the second time. A close-up view of one individual specimen is presented in Figure 8b. The presence of the black deposits on the specimens and on the PVC rack indicated that sulphide had been produced from the decay of the residual marine organisms. Energy dispersive x-ray (EDX) analysis on a sample of the black deposits showed a significant amount of sulphur, Figure 11, which further confirmed the presence of sulphide in the black deposits.


(a) 4B Front

(b) 4B Side

(c) 4B End 1 (d) 4B End 2

Figure 9: Surface appearance of Coupon 4B after 39 months of seawater exposure.


Re-deposited Cu

(a) 4B Front

Re-deposited Cu

(b) 4B Side

Re-deposited Cu Re-deposited

Cu

(c) 4B End 1 (d) 4B End 2

Figure 10: Surface appearance of 39 month exposed Coupon 4B after chemical cleaning, showing localized corrosion and re-deposited copper in and around the corroded areas.


Figure 11: The energy dispersive X-ray spectrum of the black deposit on the rack.

3.4 Metallographic Examination

3.4.1 Microstructure

Cast NAB is a complex alloy with several inter-metallic phases. The generally agreed phases include copper-rich α phase, martensitic β phase, iron-rich κI phase in rosette form, iron-rich or nickel rich κII phase (a spherical or small rosette precipitate formed at grain boundary), κIII phase (a lamellar or pearlitic precipitate forming eutectoid with α phase), and a fine needle-like iron-rich κIV phase formed within the α phases. The iron-rich κI phase is not usually present in a NAB alloy unless the iron content is higher than the nickel content in the alloy.

Microstructures associated with the specimens from different sources are presented in Figures 12 through 15. Columnar α grains were observed in the parent metal of 4A, Figure 12, while inter-metallic phases κ κ and κII, III, IV were also evident. It was noticed that some rosette precipitates were as big as 25µm, resembling the size of a typical κI precipitate; however, the rosette precipitates were still classified as κII as the iron content in the alloy was lower than the nickel content [2]. In specimen A8, as shown in Figure 13, columnar-like α grains were observed along with κ κ and κII, III, IV phases. It was also observed that the microstructures of A8 were very similar to that of 4A. This was likely due to their comparable chemical compositions (Table 1). Figure 14 shows that in Specimen B9, α phases were of columnar-like structure, but with enriched distribution of κ phases in α grains. On the other hand, there was increased distribution of κIV III phase and decreased distribution of κII phase in the alloy. In Specimen C8, as shown in Figure 15, martensitic β phase structure was observed in addition to columnar α grains and all κ phases except κI. Although the thermal history of the alloy was unknown, the presence of martensitic β phase in the alloy likely resulted from a fast cooling rate in the casting process.


Figure 12: Microstructure of Specimen 4A, parent metal.

Figure 13: Microstructure of Specimen A8.


(a) low magnification

(b) high magnification

Figure 14: Microstructure of Specimen B9.




Figure 15: Microstructure of Specimen C8.


3.4.2 Corrosion in weld HAZ

Cross section microstructures were examined on three welded specimens after 7 months, 13 months and 39 months of seawater immersion, respectively. The main effect of the heat generated by the weld is to raise the temperature in the HAZ to the point where the α phase and various κ precipitates reconstitute the high temperature β phase. The high temperature β phase, on subsequent rapid cooling, transformed into large areas of martensitic β phase in the HAZ. Figure 16a shows the cross section micrographs along the weld/parent material interface on Specimen 4A after 7 months of seawater immersion. Dark-etched martensitic β phases were evident as deep as 7.2 mm in the HAZ. No localized corrosion was observed along the HAZ; however, the inset in Figure 16a shows that defects as deep as 1.1 mm from the front surface were observed, which appeared to be micro-cracks caused by the weld. The micrograph showing the microstructure across the HAZ is presented in Figure 16b. As expected, the rapid cooling of the weld area to the right had resulted in a grain structure that was much finer than that in the parent material.

In Specimen 5A, as shown in Figure 17, dark-etched martensitic β phase was also observed in the HAZ, which reached 6 mm deep from the front surface. SPC initiated from the front surface adjacent to the weld and had developed 1.1 mm deep along the weld HAZ. One of the characteristics of SPC in NAB is the presence of re-deposited copper in the corroded areas. The close-up view of the microstructure in the HAZ near the front surface, Figure 18, indicates the presence of re-deposited copper in the area where SPC took place.

Similar to Specimens 4A and 5A, the dark-etched martensitic β phase in Specimen 4B extended 6.3 mm deep along the HAZ. On the other hand, severe corrosion occurred at the front surface in the HAZ, as shown in Figure 19. SPC further developed along the HAZ with the depth of attack reaching 6.3 mm. This corrosion depth is equivalent to a corrosion rate of 2 mm/year for the 39-month NAB specimen, the maximum corrosion rate for NAB alloys reported in literature [1]. Re-deposited copper was also observed at various depths of attack in the HAZ (e.g. Figure 20 and Figure 21).

Multi-pass GMAW was used in the preparation of the welded NAB samples, with the first pass started from the back surface. In multiple pass welding, reheating and slow rate of cooling of each succeeding pass allow time for the high temperature β phase to decompose and thus reduce the adverse effects of welding on corrosion resistance. However, the high temperature β phases reconstituted in the last few passes in the welded NAB may not have had sufficiently low cooling rates to decompose and, as a result, became martensitic β phases. This may explain why there were martensitic β phases in the HAZ near the front surface where the last few passes were applied during the welding of the NAB specimens.

3.4.3 Corrosion under crevices

All of the 13 and 39-month specimens exhibited localized corrosion at the crevices formed by the nylon washers. It was observed that crevice corrosion took place in the areas both underneath and adjacent to the nylon washers. In specimen A8, seen in Figure 22, SPC occurred inside the crevice formed by the nylon washer and the attack had penetrated approximately 0.065 mm. In the 39-momth specimen, 4B, the corrosion underneath the nylon washer resulted in a 0.66 mm deep pit, as shown in Figure 23. Crevice corrosion in the areas adjacent to a crevice formed by the nylon washer was observed on Specimen B9, seen in Figure 24. In this specimen, the corroded


area encircled the crevice former, Figure 24a, forming a ring with the attack reaching 0.28 mm deep. For the welded specimen, 5A, numerous corrosion pits were observed around the crevice formed by the nylon washer, Figure 25a. The cross section micrograph, Figure 25b, shows SPC in the pitted area with depth of pitting reaching 0.54 mm. Re-deposited copper was also seen on the surface inside the pits.



Figure 16: Cross section micrograph showing (a) weld/parent metal interface for specimen 4A, with inset showing pre-etching cross section in a selected area; and (b) microstructure across the

HAZ (after 7 months of immersion).


Figure 17: Cross section micrograph showing the weld/parent metal interface for specimen 5A (after 13 months of immersion).

Figure 18: Cross section micrograph near the front surface showing re-deposited copper in the areas where selective phase corrosion took place (Specimen 5A, after 13 months of immersion).


Figure 19: Cross section micrograph showing selective phase corrosion near the weld fusion line at the front surface for specimen 4B (after 39 months of immersion).

Figure 20: Cross section micrograph showing selective phase corrosion and re-deposited copper in the corroded areas in the heat-affected zone 4 mm from the front surface of specimen 4B (after

39 months of immersion).

Weld material

Front surface

Parent material


Figure 21: Cross section micrograph showing selective phase corrosion and re-deposited copper in the corroded areas in the heat-affected zone 6 mm from the front surface of specimen 4B (after

39 months of immersion).

Figure 22: Cross section micrograph showing crevice corrosion in Specimen A8 (after 13 months of immersion).


Figure 23: Cross section micrograph showing crevice corrosion at front surface in Specimen 4B (after 39 months of immersion).

(a) Cross section sample

(b) Cross section micrograph

Figure 24: Cross section sample and micrograph showing crevice corrosion at front surface adjoining the mouth of a crevice formed by the nylon washer (Specimen B9, after 13 months of

immersion).


(a) Front surface with marked area for cross section examination

(b) Cross section micrograph in the marked area

Figure 25: Photograph showing location of corrosion area and cross section micrograph showing crevice corrosion at front surface near the crevice formed by the nylon washer

(Specimen 5A, after 13 months of immersion).


3.4.4 Corrosion on bare surfaces

In addition to the areas under and around artificial crevices and in the HAZ of the weld, localized corrosion was also observed on the bare specimens. It had been noticed that there were significant amounts of decayed biological matter and corrosion product formed on the 13-month and 39-month specimens following the two 3-month indoor seawater exposures, e.g. Figure 8. As shown in Figure 10c and Figure 10d, the localized corrosion on most bare surfaces appeared to have been initiated under or near the crevices formed by marine growth or decayed biological matter.

The cross section micrograph on the surface at one end of Specimen C8 is presented in Figure 26. There was severe SPC at this surface with the depth of corrosion reaching 0.42 mm. Figure 27 shows that martensitic β phases appeared to have been preferentially attacked, resulting in a large number of grains that fell off the specimens. The cross section micrograph of one end surface for Specimen 4B (i.e. Figure 10c) is presented in Figure 28. Severe SPC was evident across the end surface with the maximum depth of attack reaching 0.82 mm.

Figure 26: Cross section micrograph showing selective phase corrosion at one end surface in Specimen C8 (after 13 months of immersion).

In summary, metallographic examination revealed that SPC occurred in all the NAB specimens once it was initiated after 7 months of seawater immersion; however, different corrosion mechanisms occurred depending on the metallurgical condition of each cast NAB. For “A” and “B” specimens as well as the parent metal of welded specimens, the SPC appeared to preferentially corrode the κIII phase, which was present along α grain boundaries. For welded specimens, the SPC preferentially attacked the martensitic β phases in the weld HAZ. The SPC due to the preferential attack along the martensitic β phases was found to be more severe than the SPC along the κIII precipitates. In the case of “C” specimens where martensitic β phases formed networks in the alloy, the SPC preferentially dissolved the martensitic β phase networks without


appreciable attack of the α grains. On the other hand, crevices, artificially or naturally formed, appeared to be required to initiate SPC of the NAB alloys. SPC was found to have occurred inside the crevices formed by the nylon washers and outside and adjoining the crevice formers. The 39-month specimen, 4B, exhibited SPC that extended from the area under the crevice formed by the nylon washer to the area outside the crevice former.

Figure 27: Micrograph showing preferential attack on martensitic β phase resulting in the loss of grains (Specimen C8, after 13 months of immersion).

3.5 Corrosion Rates

The average corrosion rates of all the specimens were calculated using the weight loss data obtained before and after the seawater immersion. In addition, cross section metallography was used to identify the maximum depth of localized corrosion for each specimen, and then to calculate the localized corrosion rate. The total corrosion rates were calculated as the sum of the average and localized corrosion rates. The data on the corrosion depth along the HAZ of the welded specimens was not used to calculate of the corrosion rates for the welded specimens.

The corrosion rates for all the specimens are presented in Figure 29. It is seen that the corrosion rates were negligible for the 7-month specimens, with a maximum corrosion rate of 0.005 mm/year for Specimen B7. This was anticipated since localized corrosion had not been initiated in the 7-month specimens. Corrosion rates from 0.11 to 0.52 mm/year were obtained for the 13-month specimens. It follows from metallographic examination, that SPC had been initiated in all


13-month specimens. Since there was virtually no corrosion in the first 7 months of seawater immersion, the corrosion rate after the initiation of SPC could be very high. If the period of the last 6 months was used for the corrosion rate calculation, the corrosion rate of Specimen 5A would have reached 1.1 mm/year, the highest corrosion rate reported by Rowland [5]. The corrosion rate of the only surviving 39-month specimen (4B) was measured to be 0.29 mm/year.

In the HAZ of the welded specimens, corrosion rates as high as 2 mm/year were observed. The welded specimens did not go through post-weld heat treatment. Post-weld heat treatment, e.g. annealing at 675°C for 2 to 6 hours, should restore the pre-weld structure and relieve stresses, and would be very effective in resisting SPC associated with the martensitic β phase [2].

End surface

Figure 28: Cross section micrograph showing selective phase corrosion at one end surface (i.e. Figure 10c) in Specimen 4B (after 39 months immersion).


0.0

0.1

0.2

0.3

0.4

0.5

0.6

7 13 39Time (month)

Cor

rosi

on R

ate

(mm

/yea

r)

"A" specimens

"B" Specimens

"C" Specimens

Welded Specimens

Figure 29: Maximum corrosion rates of NAB specimens after 7, 13, and 39 months of seawater immersion.

3.6 Electrochemical Testing Results

A preliminary study using electrochemical techniques was conducted to investigate the effect of decaying biological matter on the corrosion behaviour of the NAB specimens. After the eight corrosion cells were set up in two circulation loops, the system was allowed to run for three days. After the three days, the PVC rack with its residual biological matter was placed into one of the indoor water tanks. It was observed that in the subsequent days reddish scum started covering the water surface in the tank with the PVC rack. As well, the scum was observed in the four affected corrosion cells, at the water surfaces and around the electrodes. The tubing used to circulate the water became coated in a black residue. Pieces of black biological matter became entangled in the flow meter as well as in the corrosion cells. Figure 30 shows the contrast in corrosion potentials, EOC, between the four pairs of samples. The addition of the PVC rack and attached biological matter into one of the seawater tanks caused the corrosion potentials of the NAB specimens in the loop to shift considerably towards more negative potentials. The immediate shift in corrosion potentials can be attributed to the decay of the biological matter, a process that consumed the dissolved oxygen in the tank.

Figure 31 shows the variations of corrosion rates, expressed as 1/Rp, with immersion time. The addition of the residual biological matter to one of the tanks at day 4 resulted in a significant decrease in corrosion rates of the 4 specimens in Loop Two when compared to the corresponding


control specimens in Loop One. The lower corrosion rates are considered to be associated with the lowering oxygen level as a consequence of the decay of the biological matter. Throughout the remainder of the test period, the corrosion rates of the four specimens remained low and the corrosion potentials remained at the more negative potential levels, suggesting that lack of oxygen dominated the corrosion behaviour of the specimens in the remaining test period.

Towards the end of the test period, the corrosion rates of the four control specimens began to decrease, indicating the development of protective films on the specimens. On the other hand, the corrosion rates of the four specimens affected by residual marine organisms slightly increased. This was accompanied by the slight shift of their corrosion potentials towards nobler values.

In brief, the immediate effect of decaying biological matter on the corrosion of NAB was to shift the corrosion potentials of NAB towards more negative levels and decrease their corrosion rates due to the decaying process of biological matter that had consumed the dissolved oxygen in the tank. A longer term experiment (at least 3 months) is required to fully investigate the long-term effect of decaying biological matter on the corrosion of NAB in seawater.

-800

-700

-600

-500

-400

-300

-200

0 2 4 6 8 10 12 14 16Time (day)

EOC (m

V vs

. SC

E)

Loop One

Loop Two

Figure 30: Corrosion potential (EOC) as a function of immersion time (Specimens A8, B9, C8 and 5A in Loop one; Specimens A8′, B9′, C8′ and 5A′ in Loop two with PVC rack submerged in the

seawater tank after three-day testing).


0.0E+00

5.0E-05

1.0E-04

1.5E-04

2.0E-04

2.5E-04

3.0E-04

3.5E-04

4.0E-04

4.5E-04

5.0E-04

0 2 4 6 8 10 12 14 16 1Time (day)

Cor

rosi

on R

ate

(1/R

p, Ω

-1cm

-2)

8

Loop One

Loop Two

Figure 31: Corrosion rate (1/RP) as a function of immersion time (Specimens A8, B9, C8 and 5A in Loop one; Specimen A8′, B9′, C8′ and 5A′ in Loop two with PVC rack submerged in the

seawater tank after three-day testing).

3.7 Discussion on Effects of Decaying Biological Matter

Due to unexpected repair and maintenance of the acoustic calibration barge, the PVC rack with the coupons had to be moved into an indoor tank twice during the test period; each time for 3 months. Foul odour from the indoor tank was noticed after a few days of immersion. In addition, the tank water had turned black and black films were found to be distributed on the PVC rack, the rope and the NAB specimens. The presence of the black film suggests that sulphide was produced from the decay of the residual biological matter attached to the PVC rack and the rope. Significant amounts of the corrosion product observed when the PVC rack was removed from the indoor tank after each 3 months immersion suggests a higher-than-normal corrosion rate during the indoor tank immersion. The sulphide-containing black film on the specimens seemed to have acted as crevice formers and have helped to produce the conditions required (e.g. low pH in the crevices) to initiate and accelerate SPC.

In a study on the effect of decaying marine organisms on the corrosion of copper-nickel alloys in seawater, it was found that the galvanic effect between one 70/30 copper-nickel specimen exposed to gently flowing seawater and another 70/30 copper-nickel specimen exposed to decaying marine organisms resulted in accelerated corrosion of the specimen exposed to decaying


marine organisms [11]. In the current experiment, subsequent seawater exposure of the NAB specimens partially covered by the black film in Bedford Basin produced more corrosion product on the specimens. However, it remains unclear whether the galvanic effect between the surface areas not covered with the black film and the areas covered had resulted in accelerated corrosion on the areas covered by the black film. Since there are the conditions in seawater handling systems subject to repeated shutdowns that lead to potential galvanic corrosion between specimens buried in decaying marine organisms and specimens exposed to flowing seawater, further study is required to investigate this galvanic effect for NAB alloys.


4 Summary and Conclusions

A modified seawater exposure procedure had to be adopted due to unavailability of the exposure site at certain periods of time during the exposure test. This procedure provided an opportunity to examine the effect of decaying biological matter on the corrosion performance of the NAB coupons. This study led to the following conclusions:

1. Although there was a considerable amount of biological growth on the PVC rack, ropes, and nylon bolts, there was no biological growth directly attached to the NAB specimens. There was no appreciable corrosion on the NAB specimens after 7 months of immersion in Bedford Basin.

2. Excessive corrosion product was observed on the NAB specimens after these specimens were subsequently exposed to indoor stagnant tank seawater for 3 months, followed by 3 months of exposure in the basin. The excessive corrosion product was due to accelerated corrosion as a result of decaying biological matter attached on the rack and ropes.

3. The decaying organisms also produced the sulphide-containing black deposits that distributed on the rack, ropes and NAB specimens. The black film on the specimens seemed to have acted as a crevice former and helped to produce the conditions required (e.g. low pH in the crevices) to initiate and accelerate SPC.

4. SPC occurred in all 13-month and the remaining 39-month NAB specimens. SPC was found to have occurred inside the crevices and in the areas adjoining the crevices formed by the nylon washers. SPC was also found on the bare surfaces where debris of decaying marine organisms, sulphide-containing black films, and deposit of corrosion products acted as crevice formers.

5. For “A” and “B” specimens as well as the parent metal of welded specimens, SPC appeared to have preferentially corroded the κIII phase, which was present along α grain boundaries.

6. In the case of “C” specimens where martensitic β phases formed networks in the alloy, SPC preferentially dissolved the martensitic β phase network without appreciable attack of the α grains.

7. For welded specimens, SPC preferentially attacked the martensitic β phases in the weld heat-affected zone. SPC due to the preferential attack along the martensitic β phases was found to have caused more damage to the NAB alloys than SPC along the κ precipitates. III

8. The maximum corrosion rate for the 13-month specimens was found to be 0.52 mm/year. The corrosion rate for the single specimen, 4B, was 0.29 mm/year over the 39 months of seawater immersion. The maximum corrosion rate along the weld heat-affected zone was found to be 2 mm/year, which was the highest corrosion rate reported in literature.

9. The short-term (2-week) electrochemical study results show that the decaying marine organisms resulted in -400 mV shift in the corrosion potentials, and decreased corrosion rate, due to an assumed consumption of the oxygen in the seawater.


5 Recommendations

Long-term (at least 3 months) electrochemical studies are needed to quantify the long-term effect of decaying marine organisms on the corrosion performance of NAB coupons. In addition, parameters such as dissolved oxygen, pH and sulphide concentration in the tested seawater should also be measured during the experiment and correlated with the corrosion performance of NAB coupons.


6 References

[1] J. C. Galsworthy, R.S Oakley and C. A. Capley, “Recent experience with cast nickel aluminum bronze seawater system components”, in Proceedings of 2007 CF/DRDC International Defence Applications of Materials Meeting, Halifax, Nova Scotia, 5-7 June 2007.

[2] H. J. Meigh, Cast and wrought aluminum bronzes: properties, processes and structure, IOM Communications Ltd. London, 2000. Ch. 8: Mechanism of corrosion. In 1st ed., p. 156-184.

[3] G.W. Lorimer, F. Hasan, J. Iqbal, and N. Ridley, Br. Corrs. J., 21, 1986, pp.244-248.

[4] E.A. Culpan and G. Rose, British Corrosion Journal, 14, 1979, pp.160-166

[5] J.C. Rowlands, Studies of the preferential phase corrosion of cast nickel aluminum bronze in seawater, Proceedings of the 8th international congress on metallic corrosion, 1981, pp.1346-1351.

[6] Y. Wang, J.F. Porter, J. Huang, and J. Lee, Examination of VICTORIA Class Submarine Valves – Final Report, Defence R&D Canada – Atlantic Technical Report, TR 2006-285, 2007.

[7] R. Barik, J. A. Wharton, R.J.K. Wood, and K.R. Stokes, The environmental factors affecting the performance of nickel aluminum bronze, Corrosion/2004, Paper No. 04301, NACE International, 2004.

[8] R. Barik, J. A. Wharton, R.J.K. Wood, and K.R. Stokes, Marine corrosion performance of nickel-aluminum bronze. EuroCorr 2006, Maastricht, Netherlands, September 2006.

[9] K.R. Stokes, Y. Wang, C.V. Hyatt, and P.L. Mart, Environmental factors affecting the corrosion performance of nickel aluminum bronzes, Final report for TTCP Operating Assignment MAT-TP1-OA36, MAT-TP1-OA36-2007, November 2007.

[10] J. A. Wharton, R. Barik, G. Kear, R.J.K. Wood, K.R. Stokes, and F.C. Walsh, Corrs. Sci., 47, 2005, pp.3336-3367.

[11] D.R. Lenard, The effect of decaying marine organisms on the corrosion of copper nickel alloys in seawater, Corrosion/2002, Paper No. 02185, NACE International, 2002.

[12] http://www.mar.dfo-mpo.gc.ca/science/ocean/BedfordBasin/CTD_depth_profile.htm.


http://www.mar.dfo-mpo.gc.ca/science/ocean/BedfordBasin/CTD_depth_profile.htm

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3. TITLE (The complete document title as indicated on the title page. Its classification should be indicated by the appropriate abbreviation (S, C or U) in parentheses after the title.) Evaluations of Long-Term Seawater Exposure Corrosion Specimens and Electrochemical Studies of Nickel Aluminum Bronze Alloys:

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Cast nickel aluminum bronzes (NAB) are widely used in submarine seawater handling systems.However, they are susceptible to corrosion, especially selective phase corrosion (SPC), at weldsand under marine growth. This leads to high repair or replacement costs. A first step in reducingthese costs is to understand the effect of local environmental factors. In order to assess localenvironmental factors affecting the corrosion performance of NAB alloys, a long-term seawaterexposure experiment was conducted. Four sets of NAB castings, including welded specimens,were immersed in natural seawater in Bedford Basin, Nova Scotia for 7, 13 and 39 months.Corrosion performance was evaluated through visual inspection, weight loss and cross-sectionmetallographic examination. In addition, electrochemical techniques were used to study theeffect of decaying biological matter on the corrosion behaviour of NAB castings. This technical memorandum presents the analysis results of the long-term exposure experimentsincluding general corrosion and SPC in the weld heat-affected zone and under crevices formedby nylon nuts and bolts, along with local environmental conditions (dissolved oxygen,temperature, and salinity). The electrochemical study on the effect of decaying biological matteron the corrosion behaviour of NAB alloys is also presented and discussed.

14. KEYWORDS, DESCRIPTORS or IDENTIFIERS (Technically meaningful terms or short phrases that characterize a document and could be helpful in cataloguing the document. They should be selected so that no security classification is required. Identifiers, such as equipment model designation, trade name, military project code name, geographic location may also be included. If possible keywords should be selected from a published thesaurus, e.g. Thesaurus of Engineering and Scientific Terms (TEST) and that thesaurus identified. If it is not possible to select indexing terms which are Unclassified, the classification of each should be indicated as with the title.) Nickel aluminum bronze, selective phase corrosion, seawater, metallography, electrochemical technique

evaluations of long-term seawater exposure corrosion … · 2012. 8. 3. · 13 mois était de 0,52...

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