Measurement of the salt content of air at corrosion sites across Canada

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NATIONAL RESEARCH COUNCIL OF CANADA DIVISION OF BUILDING RESEARCH

MEASUREMENT OF THE SALT CONTENT OF AIR AT CORROSION SITES ACROSS CANADA

Part I : Part II:

General Discussion and Comment

Some Experiments on the Determination of Atmospheric Chlorides near Halifax

by

M.R. Foran and WoLoM. King

Report No o 122 of the

Division of Building Research

Ottawa June,

19.57

PREFACE

The Divisipn of Building Research has established eight exposure sites across Canada and maintains these for the work of the Associate Committee on Corrosion of the National Research Council. These sites, which are administered by the Divlsion$ are also used in its own projects involving exposure of

materials to representative Canadian atmospheres, and can be used by arrangement by others having need of such facilities.

It is of value in interpreting the results obtained by exposure at various locations to have, in addition to the usual climatic data, quantitative measurements on certain

other characteristics of the atmosphere such as the occurrence of sulphur dioxide and of chlorides. Routine measurements on sulphur dioxide are already being made at the various sites, but there has been no accepted simple method by which the

salt content of the air might be satisfactorily measured on a routine basiso

The Division was therefore pleased to be able to arrangeD in co-operation with the Associate Committee on Corrosion and the Nova Scotia Research Foundation for the employment of a summer student Mro WoL.M. King to work under Dr. MoR. Foran on an initial study of possible methods of measuring atmospheric chlorideso Dr. Foran's assessment of the problem together with the results of the summer9s work carried out by Mr. King

are now reported. Ottawa,

Juneg

19570

NoB. Hutcheon9

MEASUREMENT OF THE SALT CONTENT OF AIR AT CORROSION SITES ACROSS CANADA

by

ｐａｒｔＧｉｾ GENERAL DISCUSSION AND COMMENT

by

MoR. Foran

In the spring of

1956

the Nova Scotia Research Foundation arranged with the Division of Building Research of the National Research Council of Canada to have a summer research student

investigate some of the methods which might be used to determine the salt content of the air at corrosion sites where no electrical power was available and where there was no continuous supervisione Mr. W.L.Mo King, a very capable and intelligent medical student was engaged to do the worko Although he had almost no training in quantitative analysisg Mro King worked industriously at the very

tedious and exacting analytical procedures required for the

measurement of traces of chloride and mastered them well enough to obtain the results in the attached reporto Before he began his

experimental work he reviewed a great deal of the ｡ｶ｡ｩｬ｡｢ｾ

literature for methods and specific procedureso A general review of the problem and some of the requirements for its solution will be attempted here in order to show the difficulties.

A survey of the corrosion literature did not reveal any conclusive facts about the size of the salt particles which must be measured in order to relate the corrosion of metals to the salt

content of the airo The apparent salt content of air depends very much upon the method of measurement since some methods are more

suitable for large particles and others collect even the very fine particles.

The following is a list of some of the methods which have

been ｵｳ･､ｾ

10 BUbbling air through chloride-free water,

20 Drawing air through chloride-free ｦｩｬｴ･ｲｳｾ

.. 2

-4.

Exposure of wet, chloride-free cloths;

5.

Exposure of wet, chloride-free cylinders ("wet-candle" method),

6.

Determining the salt content of rainwater; 1. _{Collection of artificial dew on cold flasks,}

Ｘｾ Counting salt crystals on exposed slides;

9.

Collecting salt by impactors or jets;

10. Thermal precipitation of particles onto a glass slidse

The first seven methods all depend upon the accurate determination of small traces of chlorides by chemical methods. In each case, all surfaces, chemicals, solutions and materials must be free from chlorides at the beginning of a measurement. This requires the development of an antichloride technique for all stages of the preparation, handling, and transfer of the atmospheric chloride to the analytical vessels. These methods are therefore laborious and exacting since only a very few milli-grams of chloride can be collected unless large volumes of air or large surfaces are usede

Another factor to be considered is whether the chloride found by any of these methods comes from sodium chloride. Sugawara et al (1) and Ambler (2) found that in rainwater near the sea the' ratio of Ｈｎ｡ＫＩｾ (C1-) =

008S

t which is approximately the ratio in

sea water. In mountain fog, sea fog, inland rain, and other types of atmospheric precipitation they found that the ratio varied from

0.48

to

20430

For this reason the results should be given in terms of chloride ion alone.

The last three methods enumerated above are physical methods and t he salt particles generally have to be counted under a micro-scope. These are not suitable for the routine measurement of

atmospheric chlorideo As these methods collect ､ｵｳｴｾ spores, all types of airborne salts, and liquid droplets of various kinds it would be very difficult to isolate the chlorideso

In addition to the requirement that a Canadian method should require no electrical power and no supervision between occasional visits to the site, it must also be able to withstand

severe climatic changes, be continuous in ｯｰ･ｲ｡ｴｩｯｮｾ unaffected by wind velocity and direction, and capable of collecting all types of chloride particles which are significant for corrosion. A systematic examination of the ten methods listed above shows that no one of

3

-Bubbling air through chloride-free water is not feasible in our climate but a study of relatively non-volatile solvents for sodium chloride might be made o The lack of electrical power on the sites might be circumvented by the installation of wind-driven, six-volt generators with storage batteries floating on the lineo These would operate small blowers or pumpso

Air might be drawn through dry filters if the above-mentioned power supply system was considered feasiblee The pore

size of the filters could then be chosen to collect all slzes of particle considered significant to corrosion.

Cold tubes for the continuous collection of salt in dew hardly seem feasible where electrical refrigeration is not

available.

Exposure of wet-cloths has been found to require constant attention and they give no better results than the "wet-candle" method which requires little attention if freezing and too rapid evaporation can be avoided (2).

The exposure of moist cylinders has been found fairly suitable in the tropics (2) but no specific measures to prevent a too rapid rate of evaporation are mentioned in the literature. In order to use them in Canada they would have to be suitable for freezing temperatures o A study of antifreeze solutions in conical or cylindrical metal flasks could be made.. Glycerine has been found

(3)

to prevent undue rates of evaporation from the flasks but makes the analysis for chlorides somewhat more complicated. This method is known as the "wet-candle" method because gauze is wrapped around a tube to form a candleo The amount of chloride collected is quite small and the preparation of a chloride=free assembly is tediouso The chloride analysis is quite time consuming because of the washing of the gauze.. Atmospheric soot and grime increase the difficulty of the analysiso For general Canadian conditions the flasks and tubes should be made of Hastelloy or some non-reactive metal or of a ｰｬ｡ｳｴｩ｣ｾｬｩｫ･ ｰｯｬｹ･ｴｨｹｬ･ｮ･ｾ

The Nova Scotia Research Foundation has had dustfall or rainfall cans of standard design exposed at twelve different locations in Halifax and Dartmouth o These were generally less than half a mile from the harbouro The chloride content of the water collected during one winter month and four summer months is

shown in Table 110 For comparison Table I shows results obtained by the wet-candle method during the same summer monthso Three

stations have been selected because their exposure was comparable or identical to that of the dustfall cans.

4

-TABLE I

CHLORIDES COLLECTED BY WET-CANDLES AT HALIFAX (Quantitives are in Mgo of chloride/m2/day)

Station

_-

A B

_-

C Average

.

₊ Federal Building 16079 11 063 24052 17065-6045 York Redoubt 16024 12 065 8 059 12049-3083+ 120 3 4 13082 + Technical College 9 022 11079-2 030 TABLE II

CHLORIDES COLLECTED BY RAINFALL CANS IN ｈａｌｉｾｘ AREA

(Quantities shown 'are in Mgo chloride ion/m 7day) Station No. Dece9

2

5

June 9$6 Ju1yIJ56 Augv$6 SeptIJ

S6

1 29.,64 3078

6088

ge04 110$6 2 240$1 6081 10 022 13 00$ 3 $030 6034 7051 10,,80

4

19038 $030 6088 60 8 8

9053

5

17067

6081

7007

9096

11036

6 9 012

4078

8041 5036 7 10 026

3070

40$2 5098 11043 8 30 021 6.06 8016 8<179 13034 9

14

02$ 70$7 6061 8084

10041

10 10 083

5030

9096 11088 11088 11 9 012 6081 6 008 $093

6068

12

15039

3070

6088

6099

6016

It appears that the dustfall or rainfall cans will give essentially similar rdsults to those of the wet-candle method in coastal areas" They .ere not tried in duplicate or triplicate at the same location so the spread of the results is not known" They are much easier to handle than the wet-candles and the analysis is much easier to performe They seem to handle snow, ice and freezing conditions

very well which the present form of the wet-candle apparatus was not designed to dOe

The dustfall cans appear to be the best method to try

under the present conditions at the corrosion sites across Canada" As long as no electrical power is available at the sites it is impossible to test methods which blow or suck air through dry filters or other devices o This leaves modification of the "wet-candle" method as the only alternative among the proposed methodso So far the current research has not led to any very feasible

methods which can meet the rigid limitations imposed by the

atmospheres, the climate, the lack of power, and ｳｵｰ･ｲｶｩｳｩｯｮｾ and the need for simplicityo

REFERENCES

ｓｵｧ｡ｷ｡ｲ｡ｾ K09 S. Oana, and To Koyamae Bull o Chemo Soco Japan,

Vol o 229 Noo

47, (1949)0

(2 ) _{Ambler 9 HoRo, and AoAoJo Baine}

se,

43b,

(1955)"

Jo Applied Chem

e Vole Ｕｾ (3) King, WeL"M" Some experiments on the determination of atmospheric chlorides near Halifaxo National Research

Council, Division of Building Research, Internal Report No" 122 (Part II)j) June

1957.

6

-PART II: SOME EXPERIMENTS ON THE DETERMINATION OF ATMOSPHERIC CHLORIDES

NEAR

HALIFAX

by

More than one method of collecting atmospheric chloride for its chemical determination was considered.

A direct measurement of the chloride concentration was attempted by drawing very slowly a metered quantity of air through a tube packed with washed-glass wool o The results were unsatisfact-ory, no titratable amount of chloride being detected in the glass wool after 44.456 cUo fto of air had been drawn through the tube

over a 5 2/3-day periodo The sampler used a converted automobile heater fan, running on a 6-volt battery, as a source of powero No field trials of the unit were made. Although an appreciable

amount of chloride might be collected through the use of a high-speed sampler, one is faced with the fact that the results

obtained at one location over a period of a day or so would not be comparable to that obtained elsewhere for a ､ｩｦｦ･ｲ･ｮｴｾ and

also brief, periodo If comparison between sites is to be madeg it

is necessary to run simultaneous and fairly lengthy testse Finally, there is the problem of finding a source of power for the high-speed sampler at the various siteso

The wet-cloth method was considered to require more attention than it could be given. This methodg which consists

of exposing a cotton cloth kept wet with water or glycerine-water at 450 _{to the sea or the wind, was said by Ambler and Bain (1) to}

be in constant need of attention if the cloth were to be kept weto Another method suggested by Ambler and Bain was that of artificial dew o Dew is made to deposit on the surface of a glass container filled with ice, and the salt in the dew is then measured.

Because this method appears to collect almost exclusively the finer salt particles not deposited on metal surfaces, it would be of little value for a corrosion study involving sea-side locationso Furthermoreg since the method requires the use of low temperatures»

it would be difficult to run a test over a long-term period at those sites which have no electrical power supp1ieso

The wet-candIa method appeared to be the most promising. The wet-candles were prepared as follows o Three strips of Noo 2 gauze bandage,

3

inches wide and about 28 inches longg folded alQng the length, were wrapped around that portion of a

5

3/4-ｾ

7

-inch test-tube protruding above the rubber or cork stopper in which it was standing (Fig. 1)0 Any chloride in the gauze had been removed by boiling in distilled watero The stopper was fitted into a 500 ml wide-mouth glass flask containing 500 ml of a glycerine-distilled water solution which was 20 per cent glycerine by volumeo Two sides of the stopper were recessed a bit, allowing the gauze tails to dip into the ｧｬｹ｣･ｲｩｮ･ｾｷ｡ｴ･ｲ

solutiono The apparatus was covered with a sloping roof for

protection from rain and suno

According to Ambler and Bain, this method gives the same magnitude of results as does the wet-cloth methodo Its advantages, as far as use in corrosion research is concerned, are two. ｆｩｲｳｴｬｹｾ it is suitable for long-term tests, and

secondly, it gives an indication of the quantity of salt likely to be deposited on a surface, which is apt to make it even more useful from a standpoint of corrosion than a method which gives the absolute concentration of atmospheric chlorideo

Some modifications of the wet-candle method, as outlined by Ambler and Bain, were tested o Fiber glass tape was tried in the place of gauze bandage, but was found to be no improvement. Its chief fault was that it did not remain wet o The method, as Ambler and Bain discussed it, called for no glycerineQ Because,

without it, it is necessary to add water weekly at some sites6 it was decided to add glycerine to slow down the rate of

evaporation of the liquid in the flask and make it possible to continue the tests during winter weather, if desired, with less danger of broken flaskso _{Twenty per cent of glycerine 9 by}

volume, was found to be a suitable concentration for summer conditionso No tests were made at freezing temperatureso

In the first ｴｲｩ｡ｬｳｾ for the chemical determination of the amount of chloride deposited on the ｷ･ｴｾ｣｡ｮ､ｬ･ the contents of the flask, inclUding the ｧ｡ｵｺ･ｷｾ titrated with silver

nitratew This was thought to be unsuitable from a standpoint

of ｡｣｣ｵｲ｡｣ｹｾ a blank titration requiring too much silver nitrateo

Thus, the mercurimetric method, which had seemed promising, was triedo

In titrating mercurimetricallY9 complicating factors were encounteredo Because the end-point is represented by a faint

but permanent turbidity, colloidal organic ｭ｡ｴｴ･ｲｾ frequently present, interferedo Organic matter, ｨｯｷ･ｶ･ｲｾ along with sUlphitesg which also interfere in titrations with mercuric

nitrate, were oxidized with an excess of potassium permanganate, the excess being SUbsequently reduced with hydrogen peroxide6

=

8

=

Finally, mention should be made or the mould which appeared inside the flasks containing glycerine about two weeks after the 'flasks had been set outo Very little9 however, if anYD

was seen in each or the flasks fitted with rubber, rather

than ｣ｯｲｫｾ stopperso This mould9 the presence of which did

not prove too troublesome, obviously lived on the glycerine» the decomposition of which was indicated by the appearance of a yellow colour in the glycerine-distilled water solutiono Mould was much more prevalent with cork stoppers probably

because either a poorer fit was being obtained or the mould was originating rrom spores in the corko

The routine involved in the mercurimetric titration of the chloride collected by a ｷ･ｴｾ｣｡ｮ､ｬ･ is as follows.. Following the careful removal of the gauze strips from the ｴ･ｳｴｾｴｵ｢･ｾ

their submersion in the glycerine-water solution in the flask, and the washing of the test-tube surface into the flask D the contents or the flask were boiled.. The liquid in the flask was then poured into a beaker and distilled water was added to the gauze in the flasko After boiling, the supernatant liquid was poured into the beakero The gauze was then washed repeatedly in boiling water until all the chloride was removed o Now the

mixture that had collected in the beaker was boiled down to a volume or about 200 ml, after filtering, an excess or potassium permanganate was added and the mixture was allowed to stand for at least fifteen minuteso One ml of hydrogen peroxide was now added to reduce the excess permanganateo More hydrogen peroxide can be added if the peroxide is ｣ｨｬｯｲｩ､･ｾｦｲ･･ and there is no worry about increasing the volume of mercuric nitrate necessary

in the back ｴｩｴｲ｡ｴｩｯｮｾ but ir this is not the caseD and all of the permanganate has not been reduced9 boiling can be employedo

Boiling permanganate auto-decomposes itl' manganese dioxide acting as a catalysto Arter filtering to remove the manganese dioxide.\> the mixture was again boiled down to about 200 ml and then made up to 250 ml in a volumetric flask o Two ｯｮ･ｾｨｵｮ､ｲ･､ ml portions were taken and acidified with nitric acid (about 1 ml of

concentrated nitric acid)o

Following the addition to each aliquot of 2 ml of sodium nitroprusside indicator» and SUbsequent filtering if there was any ｴｵｲ｢ｩ､ｩｴｹｾ the aliquot was ｾｩｴｲ｡ｴ･､ slowly with standardized mercuric nitrate until a faint but permanent turbidity appearedo

A dilute 00 0 1 2 1 N mercuric nitrate solution was used, since the

chloride i11 the mixture being titrated was of such low concentration .. It was because the mercuric nitrate solution was so dilute that

2 ml of indicator were usedo It was discovered that observation

of the first sign of permanent turbidity could best be made by setting the flask on a black background and looking down into the flask through the mouth» while wrapping the hands around that portion of the flask above the level of the mixtureo

"" 9 =

Despite efforts to overcome it, unwanted turbidity persisted in some mixtures which were to be titratedo Six of twelve flasks set out on July 20 and taken in on August 10 gave acceptably close figures for the titrations of the two 100-ml aliquots" The data obtained from those six flasks are given below" An example of the method used for the single calculations is as ｦｯｬｬｯｷｳｾ With the titration figures being 3057 ml and 3054 ml for the two 100-ml aliquots drawn from the 25Q-ml total volume of ｵｮｫｮｯｷｮｾ and

0062

ml and 00 6 0 ｾｬ for the

two 100-ml aliquots drawn from the ＲＵＰｾｭｬ volume of blank

solution, the total volume of mercuric nitrate needed to react with all the chloride in the total volume of unknown

=

(3057

-

0

061 )

+ (3054 = 0061) x

5/4

=

7036

mlo

Then, ｳｾｮ｣･ one ml of 0 00121 N mercuric nitrate reacts

with ＰＰＴＲｾ mg chloride ion, the total amount of chloride ion

deposited on the gauze surface equals

7036

x 00429

=

3 016 mgo TABLE III

AMOUNT OF CHLORIDE ION DEPOSITED ON WET-CANDLES DURING PERIOD , JULY 20 - AUG0 10i 19$6₉ AS DETERMINED MERCURlMETRICALLY

Total amount of chloride iong in ｭｧｾ deposited in

21 days Mgo of chloride ionlsq meter/day

Candle A Candle B A B

Federal Building Roof 3,,16 2.86 16046 14090

York Redoubt 2,,60x lo90x l3000x 9050x

Sackville (in garden

lo27x 6035x

opposite end of paved 10 9 0 9090

Beaver Bank ｒｯ｡､ｾＩ x

Signifies a rubber stoppero If no asterisk is presentg a cork stopper was usedo

The surface area of the gauze on a test tube mounted in a rubber stopper is the sum of the surface areas of the eylinderp

89.27 sq em, and of the gauze resting on the top or in the

recesses in the side of the stopperp 5092 sq emo ｔｨｵｳｾ the total surface area is 9502 sq em or 0 000952 square meterso SimilarlY9 the surface area of the gauze when cork is used is the sum of 81032 sq cm and 10012 sq em, which equals 9104 sq cm or 0 000914 square

10

-meterse The difference found with the different types of

stoppers is due to the fact that the cork stoppers had more depth and, consequently, exposed less length of test-tube, but at the same time exposed more cork above the lip of the flask and so ｰｲ･ｳ･ｮｴ･ｾ more gauze on its own sides than did

the ｲｵ｢ｾ･ｲ stopper.

Because the mercurimetric method of titration had proved somewhat difficult, a ｦｵｲｴｨ･ﾷｾ examination of the

titration with silver nitrate was made, with pleasing resultse As with the mercurimetric method, there was extraction of the chloride by boiling, ｲ･､ｾ｣ｴｩｯｮ of the volume by boiling,

filtering, addition of an excess of permanganate

(5

ml)g standing, addition of 1 cc hydrogen peroxide, boiling if

necessary, filtering, reduction of the volume to about 200 ml by boiling, dilution to 250 mIg and the pipetting out of two 100-ml ｡ｬｩｱｵｯｾｳＮ The aliquots, however, were put in white evaporating dishes to faoilitate observation of the end-point during titration. After 1 cc of

5

per oent potassium chromate had been added to each aliquot, titration was made with silver nitratep I co of which precipitated

0.5

mg chloride ion.

The data provided by the fifteen flasks which sat out from August 10 to August 24are given on the following page.

With distance ｦｾｯｭ the sea, within the coastal area9 qUite a rapid decrease in the atmospheric content of salt

particles sUbjeot to deposition has been shown by the "wet-candle" method.

REFERENCE

(1) Ambler, HoRo g and AoAoJo Baino Jo Appl. Chem. Vol. Ｕｾ No.

11 -TABLE IV

AMOUNT OF CHLORIDE ION DEPOSITED ON WET-CANDLES DURING PERIOD AUG o 10-24, 1956, AS DETERMINED AROENTlMERTICALLYo

-Total amount of chloride ion, Mgo of chloride in mg, deposited in

14

days ion/sq meter/day

Candle A Candle B Candle C A B C

Federal Bldg. Roof. 2.15 1.55x 3014 16079 11063x 24052 York Redoubt 20 0 8 1062 1.10 16024 12 065 8059 Sackvi11e, NoS. 0.81 1.19 0.41 6033 9029 30 2 0 Mining Bldg. 1.18 1.58 1077 9 022 12034 13082 NoSoT.Co Chebucto Rd., 135' 24.28 27091 26088 189063 217098 209093 from sea

x_{denotes rubber stopper}

Mgo of chloride ion (sq meter day (average)

Federal B1dg o 17065:6049

York Redoubt 12049:3082

Sackvi11ei NoS. 6027!300S

Mining B1dgo, N.SoToCo 11079±203S

Chebucto Head Light 20S08S!14061

. Standard Deviation

=

r3=--

D2

Figure 1. "Wet Candles" exposed at NRC corrosion site at York Redoubt near Halifax.