Sensor Geometry Effects

The single most important factor in sensor performance was the geometry of the tip. The most important dimensions were the shank length, and the tip diameter, see p.vi for definition of shank length. Although sensitivity was not seen to depend on sensor geometry, the sensor response time was strongly related to both the tip diameter and the shank length, as was signal to noise ratio.

In further studies the response time and signal to noise ratio should be investigated as functions of the dimensionless tip geometry, (shank length/tip diameter). The reason this measure was suggested is that the tip dimensions were rarely the same, and this ratio would be a m o r e g e n e r a l m e t h o d to c o m p a r e the sensor characteristics. The effects of both dimensions would be amplified in this dimensionless ratio. Smaller tip diameters had larger signal to noise ratios, and larger response times, while large shanked sensors had large signal to noise ratios and slow response. Thus increasing the dimensionless tip geometry increased the sensor response time and signal to noise ratio.

Typical sensors with good response characteristics (response t:me < 5sec, S/N > 10) had tip diameters of at least 5 urn, nnd shank lengths no more than approximately

1.0 mm. Examples of typical sensor geometries are shown

on the following pages, in Figure 2: Figuie 2a (p.26) is an SEM image of a double barrel sensor tip at 6000X

magnification, the tip had been bevelled lightly.

Figure 2b (p.27) is an SEM image of a sensor's shank at BOX magnification showing a very blunt taper.

There appeared to be a critical size (about ltim)

below which sensors were very difficult to use, sensors below this size would not generally reach steady potentials, and consequently could not be calibrated. It was not clear if the ability of the experimenters was the limitation in producing functional submicron sensors. The literature (1,11) would suggest that submicron sensors should be functional. It is suggested that the fabrication process be modified slightly if submicron sensors were required in the future.

The solutions injected to both barrels must be absolutely clean. Very small particles could clog small tips creating very high resistance and thereby rendering the sensor useless. In this study the theta tubing was not cleaned prior to pulling. It was suggested by

97

uoriVDi ; ;u£)t>w X0009

Figure 2b: SEM Image of Double Barrelled Sensor Shank at BOX Magnification

Oakely (15] that cleaning, of the glass was not required as the glass at the tip would be freshly exposed when

pulled. Although at the tip this was correct,

particulate material within the theta tubing could be washed down into the tip area by the backfill solutions, again this would cause the electrode to have an extremely high resistance.

Negative* capacitance compensation instrumentation exists which may improve sensor response tame by accounting tor the charging ot the sensor glass which occurs when a current is drawn through the electrode.

For a more thorough discussion see Oakley (12).

Commerc i a 11 y available units inc 1 tide t he Axoprobe 1 i ne

from Axon Instruments. These devices are manufactured

specifically for microelectrode measurements.

B. Sensitive y

Examples of typical calibration curves are shown on the following page, figure 3, typically these curves were given by;

pH: E (mV SCE) = 500 - 59*pH with r2 0.98

Cl": E(mv SCE) = 175 - 57*pCl“ with r2 = 0.97

Typ.;-*ai >\-i i *brat n (’ ui v»> l o r Double Dm * r e i led pli 1 4 1

*.

.

*•

■

Cl lea Id I■

f* r»p

> imV r l

-1 On

l* *

4

r e p r e s e n ta tiv e , th a t is a se n so r sh o u ld n ev er be assum ed

to behave as giv en by th e s e e q u a tio n s . Every se n so r was

u n iq u e and b o th th e s lo p e and i n t e r c e p t o f th e

c a lib r a tio n cu rv es v a rie d g r e a tly from s e n s o r to s e n so r.

B ecause of th e v a r i a b i l i t y o f th e s e n s o r re sp o n se

c h a r a c t e r i s t i c s , and th e e a s e w ith w h ich th e y w ere

b ro k en i t was e x tre m e ly im p o rta n t t h a t s e n s o rs w ere

c a l ib r a t e d b o th b e f o r e and a f t e r eac h e x p e rim e n t to

e n s u re th a t th e m easu rem en ts ta k e n may be p r o p e r ly

i n t e r p r e te d .

I f d u rin g an e x p e rim e n t th e t i p o f a

s e n s o r was bumped th e c a l ib r a t io n p e rfo rm e d b e fo re th e

e x p e rim e n t was u n lik e ly to h o ld a t th e end o f th e

e x p e rim e n t, th u s th o s e u sin g t h i s te c h n iq u e m ust alw ays

be aw are o f th e ir h an d lin g o f th e s e n s o r.

The pH s e n s o rs rem a in ed s e n s i t i v e from pH*3 to

p H » ll. The c h lo rid e se n so rs had s e n s i t i v i t y to c h lo rid e

from 10~4

m

to s a tu r a te d s o lu tio n s o f N aCl. T hese v a lu e s

encom pass a l l c o n d itio n s l i k e l y to b e m easu red in

lo c a lis e d c o r r o s io n sy ste m s, th u s th e y a r e th e o n ly

r e q u ir e d pH o r C l ' s e n s o rs n eed ed to m easu re lo c a l

c o n c e n tra tio n f i e l d s .

The r e f e r e n c e b a r r e l h ad a c o n s ta n t p o t e n t i a l

re a d in g o f a p p ro x im a te ly -5 mV SCE. T h is v a lu e v a rie d

by no more th an 3 mV fo r any ran g e o f c o n d itio n s f o r any

30

s e n s o r te s t e d . T h is showed th a t th e r e fe re n c e b a r r e l

was in f a c t a fu n c tio n a l re fe re n c e p ro b e fo r lo c a liz e d

c o rro sio n system s.

The s e n s i t i v i t y ( i . e . s lo p e o f th e c a l i b r a t i o n

c u rv e s) was found to be o n ly w eakly d ep en d en t, i f a t a l l

dependent on sen so r geom etry.

C. Response Time

The se n so r re sp o n se tim e c o u ld be in te r p r e te d in

many d if f e r e n t w ays. In t h i s stu d y th e re sp o n se tim e

was found by c o n s id e rin g th e s e n s o r 's re s p o n s e to

c o n c e n tra tio n changes a s f i r s t o rd e r. Thus th e s e n s o r 's

re sp o n se tim e o r tim e c o n s ta n t was found by m easuring

th e tim e re q u ire d f o r th e se n so r to re a c h 6 3 .2 % o f i t s

s te a d y v a lu e a f t e r a s te p ch an g e in c o n c e n tra tio n was

made.

The Ag/AgCl c h lo rid e s e n so rs had v e ry f a s t resp o n se

tim e s (< l s e c ) , a lth o u g h th e s e m easu rem en ts w ere made

u sin g a system c o n s is tin g o f a r a p id ly s t i r r e d s o lu tio n

and th e change in C l' was a f f e c te d by i n j e c ti o n . The

a c tu a l re sp o n se tim e o f th e s e s e n s o rs was m ost li k e ly

s m a lle r y e t, as th e more a c c u ra te flo w in g stre a m m ethod

w ould s u re ly show.

The pH sensor response times were generally very rapid, for a five micron tip, sensor response time was - lsec. Chloride sensors were slower to respond, and typically had about twice the response time of a pH sensor for a given size electrode. The difference was undoubtedly in the membrane material used for sensing, the chloride membrane material was a liquid ion exchanger type while the hydrogen was a neutral carrier type [1]. It was not exactly clear how the different molecular str* sture of the membrane would affect the sensor response time.

The response time data show that these sensors are useful for measurements in localized corrosion systems.

In consideration of the size, sensitivity, response time, and avai l a b i l i t y of this technology, the microsensor represents the most powerful tool currently in use for local chemistry measurements in localized corrosion.

D. Lifetime

The sensors prepared in this experiment generally had lifetimes of several days once the membrane material and backfill solutions had been added. The lifetime of the sensor could of course be very different than the

observed average, so the average value should not be taken as a representative value. Each sensor was different and roust be characterized accordingly.

In this study sensors w e r e stored at room temperature and pressure in the dark. Different storage procedures would certainly produce different lifetimes.

VII. CONCLUSIONS

A. Sensor Production

This study shows that chloride sensors may be made in a routine manner from Ag wire, capillary glass and AgCl.

Double barelled pH, or C l “ sensors were produced in

a fairly straightforward way. The sensor production steps involved glass preparation, micropipette pulling, silanization, bevelling, and solution and membrane injection. Although a tedious process the sensors produced have tips on the order of microns, far smaller than any such arrangement reported in the corrosion literature.

B. Sensor Characteristics a. Geometry

The Ag/AgCl chloride sensors had tip sizes of

approximately 60 pm. The size being limited to the

diameter of the Ag wire used.

The d o u b l e b a r r e l l e d sensor's p r o d u c e d and

length was typically less than 3mm, These sizes were far smaller than those reported in the corrosion literature.

The response time and signal to noise ratio were found to be directly proportional to the dimensionless tip g e o m e t r y (shank l e n g t h / t i p diameter). No quantitative information on the form of the relationship between response time or signal to noise ratio and tip geometry was generated.