rapid corrosion of copper and brass plumbing by coastal … · inland regions to the south....

Technical Report WRD84082

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RAPID CORROSION OF COPPER AND BRASS PLUMBING BY COASTAL GROUNDWATERS OF THE NORTHERN TERRITORY

BACKGROUND

Almost all commun i ties in the Northern Terri tory re ly on groundwater in the form of bores or permanent springs as their primary source of supply. The eXCe::ltlons are Darwin, Katherine, and Ngukurr (Rocer Rive~) which have surface catchment supplies with mi~or supplementation from bores.

The underground water resources of the Territory vary greatly in quality, generally having 10· ... to very low mineralization in the monsoonal areas to the north with increas 1ng mineral i za t ion in the temperate and desert inland regions to the south.

Corrosivity of the coastal groundwater Twenty main coastal outstations having a are involved as shown

waters is variable but all the supplies are highly corrosive. cOIT@unities and numerous small total population of some 12,000 on Figure 1.

Suoolies to Alyangula and Angurugu are chemically treated to remove the corrOSlve characteristics, and aeration provides partial treatment at Nhulunbuy. All of the other supplies are reticulated without treatment other than chlorination as required.

Recently, following incidents of high water use and reports of plumbing corroding out so rapidly that houses could not be maintained in an habitable cond i t ion, the Deoartment engaged Austral ian 101 ineral Development Laboratories (AMDEL) of Queensland to investigate the problem.

The consultant concluded that failures are occurring on a massive scale to an extent unknown elsewhere in Australia, and that the high failure rate of copper pipe, brass fittings and hot water systems is a result of the high corrosivity of the soft, acidic bore waters ..

In a.ddition to house plumbing it has been observed that galvanized iron and asbestos cement reticulation mains and brass tapping ferrules and associated fittings are being attacked.

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N.T. COASTAL TOWNS WITH H!GHL Y CORROSIVE

FIGURE t

• UNDERGROUND WATER SOURCES .

5

\"

o I

I .... -----. JA81RU \ \.....

' ... \ .... KATHERINE ..... po" ....

I ... I \

"' \ ,1 \ .". '-,.", -' \

"".. -"'" \ I I \ \ I I \ \ I \ \ \

\-."--'-TENNANT CK. • "', I ... -

I /

.I { I \

" \

\ '----I I I

I

} ALICE SPRINGS

, I I I \ I

... I I

---- ALL WEATHER ROADS

km1M 0

P\."iM~

~EPT. 1984

*16 - AL Y ANGULA *18 - ANGURUGU

N

3 - BEL '(UEN (DELISSA VILLE) 20 - BORROLOOLA 12 - GALlIoIINKU (ELCHO IS.l 13 - GAPUIoIIY AK (LAKE EVEL~;. 8 - GUNBAlANY A {OENPELLIl 9 - MANINGRIDA 6 - MILIKAPITI (SNAKE BA '(1 10 - MILlNG1MBI 7 _ MINJILANG (CROKER IS) 4 ~ NGUIU (SA THHURST IS.)

*14 - NHULUNBUY 19 - NUMSULIoiAR 2 - PEPPIHENARTI 5 - PULARUMPI (GARDEN POIN-" - RAHINGINING 17 - UMBAKUMBA 1 - 101 AOERR {PORT KEATS} 15 - Y!RRKALA

I" RETICULA TED lolA TER TREATED I TO CONTROL CORROSION.

DEPARTMENT OF TRANSPORT &. WOR"


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NATURE OF CORROSIVITY

Cor-rosivity of a water may be defined as the potential or ability of that water to dissolve metals. Water corrodes exposed metal by a physio-chemical action related to the character-is tics of the water- and metal.

Waters become more cor-ros i ve wi th increas ing di ssol ved oxygen, dissolved carbon dioxide, temperature and velocity, and become less corrosive with increasing alkalinity, hardness and pf.l. Many other factors are involved but the character-istics above have the major influence on the corrosivity of waters with dissolved salts less than 1000 mg/L and chloride less than 300 mg/L, which applies in the cases under consideration.

Table 1 lists representative analyses of water from a number of the supplies involved. , The samples were generally taken at the bore head or r-eceiving tank and analysed in the laboratory up to several months later. Consequently, due to the effects of storage the results do not accurately represent the characteristics of the water as discharged from the source, particularly as regards free CO

2 in solution

and pRo These factors are interrelated and cr-itical to corros ivi ty.

Free CO 2 combines with water to H2C0 3 , ln accordanc.e with the equilbrium relationship:-

produce car-bonic acid, following dissociation

This means that increases in CO 2 concentr-ation result in increases in carbonic acid concentration and a consequent reduct ion in pH. The increase in ac idi ty due -to high CO

2 levels greatly increases the

corrosivity of the water.

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Technical R

eport WR

D84082

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TYPICIIL IlOASTIIL 9UlUlillIATERS Of TIfE II(JffiIEml TERRllURY

CORROSIVITY INDICES --LOCATION TOTAL CALCUIM TOTAL FREE

FILTERABLE AS ALKALINITY CO2 RESIDUE COC°3

AS COC03

INSOL'N

(OR TOS)

pH pHs LANGELIEH RYZNAR SAT'N STAB I L1TY INDEX INDEX pH - pfls 2plls-pfl

mg/L mg/L mg/L mg/L

\lADERA (PORT KEATS) 46 8 5 50

BElYUEN (OELISSAVILlE) 51 2 17 26

NGUIU (BATHURST IS) 28 1 4 40

MINJllANG (qROKER IS) 60 1 8 19

MANINGRIDA 40 8 2 30

RAMltlGlNING 35 0 5 55

5,2 II, 1 -5,9 17 ,0 6,1 10,2 -4.1 14.3

5,3 II , I -5.8 16,7 5,9 10,0 -4.9 15,7

5,0 11,5 -6,5 18,0 5,2 I I , I -5,9 16,8

YIRRKALA 49 2 5 4 6.3 10.7 -4,4 15,1

AtlGURUGU 25 2 27 5,1 11,4 -6,3 17,7

lJ.IBAK UMBA 60 3 37 5,2 11,2 -6.0 17,2

BORROlOOlA 36 5 50 5,3 11,0 -5,7 16,7

NOTES: W~t6r te~erl'lltur6S r~n!l0 from 21 to 300c l'lt SOlJrC6. pH 15 of smnp I e In l1)borlltory not as de II vored B t source. pHs Is tha calculated pH seturated with CtIC0

3 nt the cl\lclum lind elkblnlty of s""mple.

CO2

c~lcul.tad using I.bor.tory pH ra.dlng,

CORROSION INDICES

LANGELIER SATURATION INDEX TENDENCY Of WATER

+ 2.0 SC~16 forml~q and for practical purposos noncorrosive

---------:----:--:----;--;--,~--Slightly corrosive nnd scale-forming t 0,5

0,0 Balanced, but pittIng corrosion possible

- 0,5 Slightly corroslvQ bnd non9c~lo-form'nq

- 2,0 Serious corrosion

RYZNAR STABILITY INDEX

4,0 - 5,0 5,0 - 6,0 6 .. 0 - 1.0

1.0 - 7.5 7,5. - 9,0

9,0 and hIgher

CalMENT

-Bores. Corrosion Intolarnblo

" " " " " n

" " n

" , "

" " " Spring. Corrosion Into'erable

" " " Bores. CorrosIon Intolerable

" "

TENDENCY OF IIHER

He&vy SC,,' e

light scale

"

little sc..,le or corrosion

Corrosion significant tlOElvy corrosion

CorrosIon Intol.rable

•

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AlthQugh the laboratory pH of the samples ranges generally from 6.3 to 5.0, several field tests show that the actual pH at the bore during discharge is of the order of one unit lower. For example a test at Waderr (Port Kea::s) in 1981 gave pH values of 4.2 at site compared to 5.4 in the laboratory. At the lower pH value the calculated free CO 2 concentration is approximately 500 mg/L compared to the laboratory result of 50 mg/L. Nabalco have also reported estimated free CO 2 of approximately 500 mg/L in the Nhulunbuy bores.

An examination of the interrelationships of the above factors for potable waters indicates that a one unit drop in PH gives an approximate tenfold increase in free CO 2 , It is free CO2 in solution and its associated effect on pH which is the critical factor in the extreme corrosivity of the coastal underground waters as indicated bv Fig 2. It will be noted that for alkalinities below 50 mg/L and free CO 2 above 2 mg/L the waters are in the serious corrosion zone. consequently all the coastal underground waters are highly corrosive. This is also confirmed by the Langelier and Ryznar Indices (Table 1) which are all in the serious and intolerable corrosion range respectively.

SOURCE OF CORROSIVITY

The depth and yield of ground\>/ater resources in these areas vary according to their locality. Depths of production bores can range from 10 to 100m with the median depth being in the order of 30m. Potential yields from i 3dividual production bores can range from less than 50 m /d (0.6 Lis) up to 38000m3/d (92.6L/s) with the median yield being some 1500m /d (17.4 L/S).

A typical bore would comprise a laterite profile to about 25m (bauxite, red clay and sandy clay strata) with 'clean' sandstone over the balance of the depth.

Rydrogeologically the coastal underground water resource a!:"eas have the following points in comrnon:

(1) High seasonal rainfall; often greater than 1 metre for t.he period January t.o March inclusive. Th:'s high seasonal rainfall results in the aquifer system being recharged annually during t.he very wet hot monsoonal period above. It also results in ext.remely rapid growth of vegetation.

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•

FIG. 2

RELATIONS HIP BETWEEN ALKALINITY. CARBON DIOXIDE CONTENT AND THE CORROSIVITY OF WATER

35~--~----~--~----~--~----~--~----.

~ 30~ __ +I __ ~I ____ ~ __ ~ __ -Li __ ~ ____ ~i __ ~! ! II '

11

,1 j I I /' g 25 I-__ .+-I ---'-----f..----L1 __ -+1 __ ---l ____ +-I~ /-,l-J

~ ill I I I I/rl .. ," / c: I ~ONE O~ I ," / ~ 20 l---i~---+' -5 E-R-I-O+-U-S-C-O-R-R1-0-S-1 O-N---ij---)+I -/-/-i~r.... "aa~~-'+yl ..;----1

~ 15 ~ __ -+i ____ +-__ -+I ____ +-__ -4I __ ~'~I~~~~O~)jI~il~" __ ---j

~ 10 l-_---!! __ -+I __ ~I---i~~-~~:~::~~~~~~~~~~+v-Z-O-N-E_rIO-F--__j· Q) I I I I I A

7 ' ~~ 7 NO CORROSION e Alkalinity -Carbon D oxide Y a": V -

t1- Critical Moximui Values I ./ ./~o~/, I I 5 I ,"'TVi-" I I I

I ... / I. , Alkalinity -Carbon Dioxide

....... 1._---- . At Equ!llbrium O~ __ ~ ____ ~~ ____ -L ____ ~ __ ~I ____ ~I _____ ~I __ ~

o 25 50 75 100 125 150 175 200 Tot 01 alkalinity as Co C0<i (mg ILl

e 'I


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Tr.:ese conditions are ideal for the concentration of extreme quantities of CO in the soil air as a result of root respiration and t~e bacterial decay of organic matter. Roots and microbes change 02 in the soil air to CO 2 ,

In these environments percentages of CO 2 in soil air as high as 20 % by volume have been reported. (This compares with CO 2 values of 1 to 2% normally found in soil air and O.O~ % in the free atmosphere).

Infiltrating water takes the CO 2 into solution, part of which reacts very rapidly with the water to produce carbonic acid as discussed above. Because of the high CO

2 concentrations in the soil air and the relatively

high temperatures, this infiltrating water becomes very acidic with a pH often as low as 4.0.

(2) Aquifer systems have been developed in 'clean" sandstones where the cement binding the quartz grains is composed entirely of silica. The water infiltrating through the unsaturated zone does not increase its hardness or salinity since the sandstone it infiltrates through is devoid of any chemical substance which may be dissolved with the exception of silica Which does not affect the acidity of the water. Silica varies from 10 to 20 mg/L and is the predominant dissolved constituent in most of the

. groundwaters.

There is also little change to the chemical quality of water in the aquifer for the same reason. Thus the' chemical quality of the water in the aquifer is generally very similar to that of the original rainwater with the exception of the concentrations of dissolved CO 2 (and hence acidity) and silica.

It can therefore be concluded that the primary reason for the severe corrosivity of the coastal groundwater resources in the Northern Territory is the high CO~

concentrations in the soil air which cause a buildup or CO 2 and related acidity in the groundwater. This combined with the lack of reactive carbonate material in the strata in which the aquifer systems have been developed, has resulted in a low pH, low alkalinity groundwater.

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CORROSION EFFECTS

Bores and Bore pumps

Over the years corrosion resistant materials of construction have been introduced into the bores.

Bore casings, until several years ago, were mild steel and,. apart rrom premature 'failures particularly at the water/air interrace, caused troublesome iron levels in some supplies as corrossion occurred. Fibre reinforced plastic (FRP) casings are now used together with stainless steel screens.

Galvanized steel tube columns were found to last only 12 to 24 months. Consequently they are now constructed of FRP. Adaptors to the pump and header are in 316 stainless steel.

Galvanized steel bore headers only last about 12 months. investigated but will probably

are still in use but they Alternatives are being

be FRP.

Submersible pumps are typically in all 304 stainless steel (Grundfos), zinc free bronze/stainless steel (Giles and Gaskin) and stainless steel casings, bronze or plastic impellers (Southern Cross).

In shaft driven pumps only are used with h 19h tens i le spiders, stainless steel stainless steel rotors and pump casings.

Tanks

Mono progressing cavity units steel sharts, moulded rubber bushes, chrome pI a ted 316 heavy duty' galvanized steel

Generally southern Cross squatter tanks are used comprising O. 6mm hot dip galvanized sheeting with Marviplate bonding both sides and neoprene rubber sealed joints. Internal hot dipped galvanized angle bracing and roor supports are used.

A FRP tank has been installed at Gunbalanya (Oenpelli) and is giving good service.

Galvanized steel tanks generally corrode out in about 5 years.

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Transfer pumps

Cast iron housings and impellers are used with stainless steel shafts. The CI impellers have been found to give satisfactory performance and the more corrosion resistant bronze and stainless steels have not had to be used.

Reticulation Pipes

In the older systems galvanized iron piping was used which suffered severe tuberculation. The water quality detioriated due to suspended and dissolved iron corrosion products. Pressures dropped due to restricted pipe bores.

Asbestos cement pipes have been used in :nany localities. Some cases of considerable corrosion by leaching of the cement from the walls have occurred. However, because system pressures are relatively low no large scale failures have occurred. A cheCK of the fibre content of affected supplies has shown that the total fibre content from all sources, not just asbestos cement attack, is less than one tenth of the WHO limit.

Cement lined ductile iron, cast iron and mild steel pipes have not been used to any great extent.

PVC pipes have been used almost exclusively in recent years for the more remote communities because of their ease of transportation and handling. No corrosion of these pipes has occurred or is ,. .

expec~eQ.

One effect of the galvanized iron, asbestos cement and cement lined pipes has been the reduction of aggressiveness of the water on passage through the reticulation due to reaction with the pipe materials. This has resulted in a reduction of the acidity of the water entering house plumbing systems and a consequent reduction in the copper and brass plumbing corrosion failure rate. Introduction of PVC reticulation pipes has resulted in vi=tually no reduction in corrosivity from bore to house, thus aggravat ing the plumbing corros ion problem.

An example of this effect occurred at Waderr (Port Keats) where due to low water pressure in the old GI/AC reticulation system a PVC ring main was installed. Within 6 months of the new installation, coincident failure of copper house services occurred which had given satisfactory service £or some yea=s.

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The 110sp i tal was connected between the old and new lines and the section fed by the GI/AC reticulation had copper leve Is of less than 0.5 mg/L whereas the sect ion fed by the new PVC main had copper levels up to 12 mg/L.

House Plumbina .

Most houses have PVC from main to building and hot and cold water plumbing in copper tube and brass fittings. The aggressiveness of the water has resulted in rapid carras ion by cuprosolvency, eros ion corros ion of copper and dezincification of brass.

Cuprosolvency is simply dissolution of the copper into the water and usually results in the familiar blue/green staining of sinks, showers and toilet bowls.

Erosion corrosion of copper tube occurs by the removal of metal from the pipe surface as dissolved ions or the sweeping of solid corrosion products from the surface by the flow. The effect is accelerated at locations of. turbulence such as at bends. It is Characterised by the presence of grooves, waves, rounded holes and usually shows a directional pattern.

Dezincification is the selective removal under acidic cond i tions of z inc from brass which generally conta ins around 30% zinc and 70% copper. The commonly accepted theory of dezincification is that the brass dissolves, the zinc ions stay in solution and the copper, being more noble, plates back on. The result is a porous copper structure having little strength.

Standard grade brass tap seats suffer rapid failure by grooving, considered to be due to a combination of initial dezincification and then erosion corrosion of the residual porous copper structure.

Corrosion of galvanised iron service pipes has occurred as for reticul.ation mains mentioned above.

Hot Water Services

Premature failures of solar collectors and hot water storage· tanks are a regular occurrence. Cuprosolvency attack on the copper pipework and dezincification corrosion of brass fittings appear to be the main causes.

CORROSION CONTROL MEASURES

Chemical Treatment

GEMCO carries out sodium hydroxide dosing to neutralize the acidity in the Alyangula and Angurugu supplies.

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Slak"ed lime dosing was attempted some 10 years ago at Maningrida but ",as discontinued due to the difficulty of operating and ~aintaining the equipment in the remote location.

A further difficulty .with all<ali dosing is the relatively small dose required to condition the water to a noncorrossive state without causing scale forming tendencies.

operation, plants in for some not been

Because of the difficulties of manning, maintenance and monitoring of chemical dosing remote corrunun i ties often inaccessible by road four months of the year, this alternative has adopted.

This decision precludes bicarbonate dosing of the supplies which is being proposed by the Hunter District Water Board and CSIRO as a means of controlling cuprosolvency and dezincification problems in its supplies and a number of others in NSWand Victoria.

Aeration

Aeration was tried at Maningrida for a short time and is used at Nhulunbuy by Nabalco. It appears to increase the pH by 0.5 to 1.5 units. For example at Nhulunbuy fan assisted updraught aeration results in the pH rising from approxima tely 5.4 to 6.8 and a reduction of the free CO2 from 44 to 3 mg/L. .

_"eration is not proposed for the supplies at this stage but may be adopted in the future in order to raise the pH to within the current Australian Department of Health desirable range of pH 6.5 to 9.2.

Introduction of aeration would involve replacement or enlargement of existing bore or transfer pumps due to the additional heads, or introduction of aeration units complete with lift pumps. Considerable expense would be involved in either case.

pumps, Tanks and Reticulation Mains

Corrosion resistant materials which have been progressively introducted in recent years as outlined above are proving satisfactory but, as previously mentioned, are exacerbating the house plumbing corrosion problem.

House Plumbing

It is proposed to eliminate galvanized iron; copper and standard brass tubes and fittings from house plumbing and to substitue corrosion resistant components.

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The following materials are generally proposed:

1. Tapping saddles to be reinforced plastic dezincitic3tion resistant (DR) brass ferrules.

with

2. The service pipe from the main to the house to be Class 12 PVC.

3. Meters to be dezincification resistant and located in a precast box underground with a DR brass meter cock. Burial is to obviate the PVC upstands which would be susceptable to damage.

4. Hot and cold pipework within buildings to be polybutylene (PBl with compression union type joints.

5. Combinations, tap bodies and gate valves to be in DR brass

6. Copper free hot water services.

Reinforced ~lastic tapping satisfactorily for some time pipe.

saddles have ben used as has Class 12 PVC service

Because of the high corrosivity of the waters it is not known at this stage if DR brass to .AS 2345 will be adequate. However, until experience shows otherwise, it will be used for combinations and tapware but installed to facilitiate replacement should it become necessary.

One point which has been noted is that the DR properties of brasses are destroyed by overheating. Consequently it is proposed that joining eny brasses to PB or PVC be totally by mechanical means without brazing.

PB svstems to AS 2642 are not yet manufactured in Australia to the best of our knowledge. Bowever, until they become available approval will be given to New zealand suppliers to provide equivalent products.

The several PB jointing systems available have studied and it is propsoed to use compression union fittings. The advantages of this type of joint are to be:

been type seen

1 It provides for full bore discharge, Le. no restriction at joints or in fittings.

2 There are no metal parts in contact with the water. 3 It does not involve a crimping tool subject to operator

variations. 4 It can be easily

modifications.

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PB has a Durometer hardness of 60 which .is just within the termite attack range. Consequently, it is only to be -installed within buildings and not under slabs or from main to building. At this stage embedment in concrete will not be permitted. However, it is possible that after experience is gained embedment will be allowed because PB is understood to perform satisfactorily in such situations.

Tapware in DR brass with ceramic seats is to be permitted.

Hot water Services

Development of corrosion resistant solar and electric hot water services has been given over to the manufacturers

Solarhart have a jacketed solar unit in which the fluid recirculating in the collector is separated form the water being heated. It uses a heat exchanger jacket to heat the water. The exchanger has a vitreous enamel lining with anodic protection. The unit has potential but costs some $600 more than a standard unit.

It is understood that Beasley are in the process of developing a fully stainless steel solar unit.

Additional Costs

The current average overall water and drainage plumbing cost for a house is about $ 4000. When the PB/DR brass system is fuly developed and ref-ined, it is estimated that an additionial cost of some $600 or 15% apply.

preliminary extra cost estimates for some Borroloola houses soon to be constructed are as follows:

Solarhart jacketed HWS Valves and taps Transfer of plumbing runs to ceiling

Additional Cost

600 400

1 000

$2 000

This represents about 50% of the current average plumbing cost.

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CONC>.USIONS

I The extremely high corrosivity of the NT coastal underground watet"s is due to high concentt"ations of free CO2 in solution coupled with vet"y low alkalinit1es.

2 Corrosion of galvanised iron, copper and brass plumbing has been accelerated by the progres:ive introduction of corrosion resistant materials 1nto bore equipment, pumps, tanks and reticulation mains 1 in part icular PVC ret icula t ion mains. Prior to this, reaction with corrodable materials in general, significantly reduced the aggressiveness of the water to household plumbing.

3 Alkali dosing of the systems to condition the water is not considered feasible having regard to operational difficulties related to community isolation and remoteness factors.

4 A polybutylene (PBl dezincification resistant (DR) brass system wi thin buildings has been adopted as the best corrosion resistant plumbing option.

5 The development of corrosion resistant solar electic hot water services rests largely with manufacturers.

N.T. DEPARTMENT OF TRANSPORT AND WORKS

SEPTEMBER 1984

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rapid corrosion of copper and brass plumbing by coastal … · inland regions to the south....

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