PRODUCCIÓN DE TEXTO EXPOSITIVO DESCRIPTIVO

4 6.30 i Non-exposed Iron carbonate Loose Corroded 5 i 6.30 Exposed Iron carbonate Strong 1

Did not corrode

6 6.30 : Exposed Iron

carbonate

Strong 1 Did not corrode

7 6.30 ; Non-exposed Iron carbonate Loose ! Corroded 8 6.40 1 Non-exposed Iron carbonate Loose Corroded

• Non-exposed means the specimens that were not taken out during the whole experiment so were not exposed to oî^gen.

Apart from testing the specimens with water some of the specimens that were regularly taken out during weighing (and had already showed good protection in distilled water) were immersed in Super Clark solution (pH adjusted to -2.00). It took around 24 hours for this solution to dissolve the iron carbonate scale and start corroding the steel substrate. Further experiments showed that the iron carbonate produced through precipitation from solution was readily dissolved by this reagent.

3-5-Stirring effect: Though during this project no specifically designed experiment was conducted to investigate the effects of stirring on nucléation and growth of iron carbonate crystals, nevertheless some related observations were made during precipitation, spectrophotometry, corrosion and galvanostatic experiments. It seemed that stirring (or sometimes solution agitation via bubbling CO2 gas into it) had two opposite effects on the nucléation and growth of iron carbonate crystals. The first one

was a positive effect and the second one was a negative or destructive effect as discussed below.

3-5-1-Corrosion experiments: During corrosion experiments it was observed that areas of the steel specimen whose access to solution was partly blocked (for example areas blocked by the hanging plastic wire) had not grown iron carbonate and the specimen had suffered from corrosion at those areas (Fig. 3-23).

Fig. 3-23-The blocking effect of the plastic wire used to hang the specimen. Solution pH=6.30, T=75C, in BP solution and after 24 hours.

On the other hand it seemed that areas of the specimen that were either located too

close to the stirring source (the glass tube bubbling CO2 into the solution) did not grow

a scale. Furthermore, at times when the stirring effect was intensified no iron carbonate scale formed on the specimen surface.

3-5-2-Cyclic voltammetry experiments: All of the reported cyclic voltammograms have been acquired under static conditions. In few occasions when the CO2 bubbling (the source of stirring in the system) was left on unintentionally iron carbonate did not form on the working electrode and the voltammograms looked exactly like Fig. 3-2la, causing a severe corrosion of the working electrode.

3-5-3-GaIvanostatic experiments: During these experiments iron carbonate did not precipitate when the working electrode was affected by the solution stirring. On the other hand iron carbonate precipitated at the working electrode when another disk with

a hole at the middle of it was placed under the working electrode disk (providing a region o f static solution around the working electrode), hence reduced stirring effects at the working electrode.

3-5-4-Spectrophotometry experiments: It was observed that stirring the solution by a stirrer improved the distribution of Fe^^ ions (shortly after their introduction into the system), resulted in better mixing of them with carbonate ions already present in the solution, hence faster precipitation kinetics. The experiments during which the solution was stirred had shorter induction times compared to ones that were not stirred at aU.

3-6-Additive effects (scale inhibitor effects)

3-6-1-Spectrophotometry experiments: All the scale inhibitors used (including both phosphonate- and nitrogen-based) inhibited (or retarded) the nucléation and growth of iron carbonate crystals at 10, 50, 100, 500 ppm concentrations within the period of all experiments (approx. 2 hrs). Figure 3-24 shows the effect of ATMPA scale inhibitor. As can be seen, no precipitate formed within the timescale of the experiment. The same behaviour was observed for the other 5 scale inhibitors.

I N20@10 ppm ATMPA 2000 3000 4000 5000 IN 21 @ 50 ppm ATMPA IN 22@ 100 ppm ATMPA 2000 3000 4000 5000 7000 0000

I ...

Fig. 3-24-ECTect o f various concentrations o f ATMPA scale inhibitor on precipitation as measured by turbidity o f the solution, no precipitation was observed. The other five scale inhibitors had the same effect, as the absorption remained fiat for all. All at pH=6.30, T=75 C, solution 0.5M NaCl.

3-6-2-Corrosion experiments: As said earlier it is of paramount importance to find a scale inhibitor that would prevent the nucléation and growth of all types of scale except for iron carbonate. All the scale inhibitors studied prevented nucléation and growth of FeCOs from homogeneous solution. However, in the corrosion experiments the situation was different. Among six scale inhibitors used, three seemed to have not been interfering with iron carbonate precipitation at certain concentrations. Three of the inhibitors, those which inhibited iron carbonate growth, were phosphonate-based, whilst the other three, which were much less influential on the iron carbonate system, were nitrogen-based. While tripolyphosphate (TPP), amino-tris(methylenephosphonic acid) (ATMPA) and phosphonic acid (PA) inhibitors completely suppressed iron carbonate nucléation during corrosion experiments at all concentrations, polyethylenimine (PEI) did not prevent iron carbonate precipitation at 10, 50, 500, and 1000 ppm concentrations; the specimens were covered with a patchy iron carbonate scale. At 100 ppm concentration it seemed that there was not any inhibition and the scale had covered the specimens uniformly and completely (Fig. 3-25). This was more or less the same for poly (diallyldimethylammonium chloride) (PDAMA); at 10, 500, and 1000 ppm concentrations a patchy coverage was obtained while at 50 and 100 ppm concentrations coverage and morphology was like that observed when no scale inhibitor was used.

#

5 8 0 0 1 9 20KV X 5 . 0 0 K " 6 * è u »

Fig. 3-25-Precipitated iron carbonate in the presence of polyethylenimine (a) and poly (diallyldimethylammonium chloride) (h). Both at 100 ppm concentration, pH=6.30, T=75C, and in

BP solution after 24 hours.

The other nitrogen-based inhibitor used was polyaspartic acid (PAA), which inhibited scale formation at lower concentrations (10 and 50 ppm) but let iron carbonate produce a thin film at 100 ppm and higher concentrations (Fig. 3-26).

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b)

t i a

580 0 3 6 20KV X i:6 & K "'! ^Â L

Fig. 3-26-Morphology of the iron carbonate crystals precipitated at 100 (a) and 500 ppm concentration o f polyaspartic acid (b). Both at pH=6.30, T=75C, and in BP solution after 24 hours.

For the three phosphonate-based inhibitors it seemed that at 500 ppm concentration the specimen surface was covered by inhibitor him that was cracked for all 3 of them (Fig. 3-27). For the three phosphonate-based inhibitors at 1000 ppm concentration the steel surface was partly covered by the inhibitor him and partly corroded.

a) bBBnKBa b)

578031 20KV &KWk'IW#u #

c)

Fig. 3-27-The inhibitor film on the steel surface for FA (a), ATM PA (b), and TPP (c), all at 500 ppm concentration. All at pH=6.30, T=75C, and in BP solution after 24 hours.

Based on their efifect on iron carbonate precipitation and morphology we can arrange these compounds as below (starting with the one having least efifect):

PEI > PDAMA > PAA > TPP, ATMPA, PA

This shows that phosphonate-based scale inhibitors which were originally designed for calcium carbonate polymorphs also proved to be efiScient inhibitors for iron carbonate system. On the contrary nitrogen-based inhibitors did not prevent iron carbonate crystal growth at almost all concentrations as at least a thin or patchy coverage of iron carbonate scale was obtained.

3-6-3-Cyclic voltammetry experiments: The scale inhibitors used were polyethylenimine (PEI), poly (diallylmethylammonium chloride) (PDAMA), and amino-tris (methylphosphonic acid) (ATMPA). The hrst two of these inhibitors were nitrogen-based and the latter was phosphonate-based and the concentrations used were

1,10, 50, and 100 ppm. Regarding the frequency of the breakdowns and the appearance of spikes it was evident that the scales grown at 100 ppm of PEI and PDAMA and 50 ppm of ATMPA were the more stable (Fig. 3-28). Figures 3-29 and 3-30 illustrate the variation of corrosion current and 0 (definition section 2-2-6-1) as a fimction of scale inhibitor concentration.

3-6-3-1-Poly (diallylmethylammonium chloride) (PDAMA): The only notable effect of this scale inhibitor regarding the electrochemical data presented in Table 3-6 was that the corrosion potentials and the corrosion rates obtained at 1 0 0 ppm concentration of this scale inhibitor were lower compared to the other concentrations. The corrosion potentials for the other three concentrations appeared to be very close for both sweeps.

Table 3-6-E!ectrochemicaI corrosion data for PDAMA scale inhibitor, all at pH=6.30, T=70 C, solution 0.5M NaCl, all potentials vs. Ag/AgCI ref. electrode (potential= 45 mV vs. SCE).

Rp: kohm, Lorr: mA/cm^, Pc: mV/dec, Ecorr : mV

ppm ; 0 ' ip(mA) : Vp(mV) ; Forward Sweep Backward Sweep

I R p ; ■ p ^corr 'i icorr ;_{- „ P a J} D o : F ^corr I_{! J ^ J} Pc j 1 0.47 i 0.76 1 -16 ' 13 ! -616 ; 94 ; 23 i 13 ! 10 ; -458 : 86 i 34 ; 12 : L i o _ j 0.58 : 0.99 ; 71 ! 9.3 ; -614 i 112 ; 27 ! i3_; -454 1 104 1 36 _{Î J I j}

50 : 0.61 1 0.57 ;_{[_±}_ _ _ ! 12 : -586 : 116 i 28 i 9 i 7.5 I -445 1 129 I 37 : 10 ;

100 i 0.74 ! 0.86 i 140 ; 16 ! -644 : 46 : 22 i 21 1 21 -499 i 43 i 28 : 14 i

3-6-3-2-Polyethyleiiimine (PEI): Like the other nitrogen-based inhibitor already discussed the corrosion potentials for the 1 0 0 ppm concentration were the lowest for both sweeps. For the first three concentrations the corrosion potential decreased by increasing inhibitor concentration. The lowest corrosion rates observed were for the 50 ppm concentration followed by the 100 ppm concentrations (Table 3-7).

Table 3-7-EIectrochemical corrosion data for PEI scale inhibitor, all at pH=6.30, T=70 C, solution 0.5M NaCl, all potentials vs. Ag/AgCl r e f electrode (p o te n tia l 45 mV vs. SCE).

Rp: kohm, icon-î mA/cm^, p»» Pc: mV/dec

ppm ; 0 i i p ( i n A ) 1 Vp(mv) ; L F o ™ r d S ; ^ ___ Backward Sweep

J i _{Ecorr i Wr 1 Pa :_J3c_.j Ecorr i icorr t Pa :L & j} 1 0.03 j 0.72 ! 7.6 10 : -642 ; 70 j 35 : 15 : 6.7 j -479 : 51 1 88 12 1 10 i 0.01 Î 0.68 j -40 _{i 18 : -644 I 77 : 15 i 17 1 11 ! -490 1_ 2 U . 96 1 15 i} L 5 0 _ J 0.03 i 0.72 ! 47.3 I 3 i -788 i 51 ; 20 ; 1 2 ! _{65 1 -572 iL M J 79 1 41 ;} 100 ; 0.02 { 0.87 1 120 _{! 3 ; -545 i 157 1 44 ; 19 ; 18 ! -405 1 193 ! 68 : 15 i}

3-6-3-3-Amino-Tris (methylphosphonic acid) (ATMPA): This phosphonate-based scale inhibitor had the lowest corrosion rate at 50 ppm concentration and the lowest corrosion potentials obtained also were associated with this concentration. The supersaturation o f the system was the same for the first three concentrations and only decreased slightly at 100 ppm concentration (Table 3-8).

Table 3-8-Electrochemical corrosion data for ATMPA scale inhibitor, all at pH=6.30, T=70 C, solution 0.5M NaCl, all potentials vs. Ag/AgCl ref. electrode (potential= 45 mV vs. SCE).

1^: kohm, icorr: mA/cm^, p „ pc: mV/dec, Ecorr: mV

ppm ₈ lp ( m A ) : V p ( m V ) ! Forward Sweep Backward Sweep

R pi Ecorr Icorr 1 Pa ; Ecorr : icorr j Pa ;_{.A _ :}

1 0.55 i 0.75 ; -47 ₆ : -596 ; 132 ; 33 ! 13 1 6.5 ! -455 ; 135 i 38 1 11 i

J O J 0.08 i 0.75 I -8.2 : 20 ! -662 : 72 i 21 ^ 12 : 10 ; -494 ! 38 i ₆₁ i 15

50 0.18 ! 0.74 ' -24 . 18 I -681 : 38 : 22 '_ 2 3 j 26 ! -514 ! 17 1 53 : 15

100 ppm ATMPA 100 ppm PEI 100 ppm PDAMA 14 - ? 1 2 -■

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