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La morfologıa y composición de los recubrimientos se analizó por medio de microscopia electrónica de barrido y la sonda de análisis de dispersión de rayos X.

Revista Mexicana de Ingenier´ıa Qu´ımica Academia Mexicana de Investigaci´ on y Docencia en Ingenier´ıa Qu´ımica, A.C.

Revista Mexicana de Ingeniería Química Volumen 12, N´ umero 1, Abril 2013

ISSN 1665-2738

Vol. 12,CONTENIDO No. 1 (2013) 129-141

1

Volumen 8, número 3, 2009 / Volume 8, number 3, 2009

THE EFFECT OF OPERATING CONDITIONS DURING PLATING ON THE ELECTROCHEMICAL BEHAVIOR AND MORPHOLOGY OF TRIVALENT SOLUTION-DERIVED CHROMIUM COATINGS

1

213EFECTO Derivation and application of the Stefan-MaxwellDE equations EL DE LAS CONDICIONES FUNCIONAMIENTO DURANTE EL ´ CULTIVO SOBRE EL COMPORTAMIENTO ELECTROQUIMICO Y LA (Desarrollo y aplicación de las ecuaciones de Stefan-Maxwell) ´ MORFOLOGIA DE LOS RECUBRIMIENTOS DE CROMO TRIVALENTES Stephen Whitaker ´ DERIVADOS DE SOLUCION

O.J Suarez1 , J.J Olaya1∗ and S. Rodil2 1

Biotecnología / Biotechnology

Universidad Nacional de Colombia, Bogot´a, Colombia. Ciudad universitaria edificio de posgrado de materiales. Bogot´a-Colombia. 245 Modelado de la biodegradación en biorreactores de lodos de hidrocarburos totales del petróleo 2 Instituto de Investigaciones en Materiales, Universidad Nacional Aut´onoma de M´exico, M´exico D.F.-Mexico. intemperizados en suelos y sedimentos

Received 28 of February of 2012; Accepted 17 of January of 2013

(Biodegradation modeling of sludge bioreactors of total petroleum hydrocarbons weathering in soil

Abstract and sediments)

Trivalent electrolyte-based chromium plating represents an alternative for the major problems associated with traditional S.A. Medina-Moreno, S. Huerta-Ochoa, C.A.chosen Lucho-Constantino, Aguilera-Vázquez, A. Jiménezhexavalent plating. Response surface methodology was in this work forL.designing experiments. Chromium coatings were obtained from trivalent solutions in a chloride-based bath using different operating conditions. Three concentrations (0.19, González y M. Gutiérrez-Rojas 0.38 and 0.57 mol L−1 as Cr(III) ), three pH (2.0, 3.0 and 4.0) and temperature (25◦ C, 32◦ C, 40 o C) was varied during the 259 Crecimiento, sobrevivencia y adaptación de Bifidobacterium infantis a condiciones ácidas experiments. Scanning electron microscopy and energy dispersive X-ray analysis were used for examining the coatings surface morphology(Growth, and composition; method was used forinfantis thickness measurement and potentiodynamic and impedance survival the andcoulometric adaptation of Bifidobacterium to acidic conditions) tests on 3% NaCl were made on a three-electrode cell for studying chromium film electrochemical behavior. Large differences L. Mayorga-Reyes, P. Bustamante-Camilo, A. Gutiérrez-Nava, E. Barranco-Florido y A. Azaolain morphology, thickness and corrosion resistance were observed, mainly due to variations in chromium concentration and pH Espinosa solution. Crack patterns were observed on films obtained at high pH values independently of process temperature.

265 Statistical approach toplating, optimization of ethanolcorrosion, fermentation by Saccharomyces cerevisiae in the Keywords: trivalent chromium, microstructure, electrochemical impedance.

Resumen presence of Valfor® zeolite NaA Los electrolitos basados en el cromo trivalente presentan una alternativa a los problemas asociados con los recubrimientos estadística de la fermentación etanólica de Saccharomyces cerevisiae endepresencia tradicionales(Optimización basados en cromo hexavalente. En este trabajo se empleo un dise˜no de experimentos superficiedede respuesta para observar el efecto de las variables. Los recubrimientos de cromo se obtuvieron a partir de soluciones de cloruro de cromo zeolita Valfor® zeolite NaA) modificando las condiciones de electro deposici´on. Tres concentraciones (0.19, 0.38 y 0.57 mol L−1 de Cr(III) ), tres valores de G. Inei-Shizukawa, H. A. Velasco-Bedrán, G. F. Gutiérrez-López and H. Hernández-Sánchez pH (2.0, 3.0 y 4.0) y tres temperaturas (25◦ C, 32◦ C y 40 o C) se variaron durante los experimentos. La morfolog´ıa y composici´on de los recubrimientos se analiz´o por medio de microscopia electr´onica de barrido y la sonda de an´alisis de dispersi´on de rayos X acoplada al microscopio; el espesor de losengineering mismos se midi´o por titulaci´on coulombim´etrica y el comportamiento electroqu´ımico Ingeniería de procesos / Process a la corrosi´on se estudio por medio de polarizaci´on potenciodin´amica y espectroscopia de impedancia electroqu´ımica en NaCl 271 Localización de una planta industrial: Revisión crítica y adecuación de los criterios empleados en 3%. Se observaron grandes diferencias en la morfolog´ıa, espesor y resistencia a la corrosi´on debido principalmente a la variaci´on esta decisión en la concentraci´ on de cromo y al pH de la soluci´on. Se observar´on patrones de agrietamiento cu´ando el deposito se obtuvo a altos valores(Plant de pHsite independientemente la temperatura del electrolito. selection: Criticaldereview and adequation criteria used in this decision)

Palabras clave: cromo R.L. trivalente, microestructura, corrosi´on, impedancia electroqu´ımica. J.R. Medina, Romeroelectro y G.A.plateado, Pérez

∗ Corresponding

author. E-mail: [email protected]

Publicado por la Academia Mexicana de Investigaci´on y Docencia en Ingenier´ıa Qu´ımica A.C.

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1

Introduction

Modern society’s population growth and industrialization have resulted in improvements in people’s quality of life, but the cost to be paid for this has resulted in a major environmental impact on air, water and soil. Chromium deposits obtained by electrolysis are of great importance in the field of decorative and industrial coatings; however, the conventional process using hexavalent chromium solutions presents serious drawbacks from environmental and occupational health view-point (European Commission, 2006), that fact has causes to different physicochemical alternatives have been proposed to deal with Cr(VI) solutions (Barrera et al., 2003, 2004). Alternatively, processes have been based trivalent chromium chemistry to reduce some hexavalentchromium associated problems. They have not fully implemented in an industrial environment because of such technology’s higher maintenance cost and incomplete knowledge (Suarez, 2006; Suarez et al., 2009). Chromium metal cannot be obtained directly from trivalent chromium sulfate or chlorides solutions, since they form stable and inert complexes with [OH]− , [SO4 ]−2 , [Cl]− ions, and oxides during electrodeposition interfering cathodic reaction. Chromium should then be used as a complex with a compound (ligand) that can easily glide the Cr(III) ion and reduction to Cr(s) thus occurs (Szyncarzuk et al., 1988, 1989; Song and Chin 2002; Vinokurov et al., 2005). The main complexing agents are: sodium hypophosphite and glycine, as well as other chelating agents such as carboxylic acids or salts, urea, thiourea, thiocyanates, dimethylformamide, hydracine or hydoxilaminophosphates (Baral and Engelken, 2005; Surviliene et al., 2007; Boasong, 2006). Also they found that using chromium chloride solutions with formates and acetates as complexant agents, obtainig chrome surfaces and experimentally showing that complexing agents were actively involved in Cr(s) reduction reactions and the elements presents as compounds in the solution, such as oxygen, carbon and phosphorus are incorporated into the coating, thereby modifying its structure. Other researchers (Surviliene and Nivinskiene, 2006) have studied the effect of operating conditions (i.e. pH, temperature, current density and adding agents) on cathodic efficiency, morphology and structure of the deposits; finding is a strong response variable dependence in all cases. 130

For example, the effect of reaction conditions during Cr(III) complex formation, as well as the effect of compounds present in solution, on chromium film morphology and corrosion resistance have already been reported (Suarez, 2006; Suarez et al., 2009; Suarez, 2010). These studies have indicated that a mixture of sodium formate plus sodium acetate with the addition of boric acid and sodium lauryl sulfate (with no other salts as additives) has shown better performance regarding corrosion pattern. The electrochemical behavior of chromium films obtained at different values for chromium concentration, pH and temperature during plating was studied in this work through a response surface experiment. This kind of experiment has been used for examining the effect of operating conditions during plating in trivalent and hexavalent baths (Baral and Engelken, 2005; Bayramoglu, 2008). Potentiodinamyc polarization and electrochemical impedance spectroscopy (EIS) were used to study the chromium deposits electrochemical behavior. Morphology and composition were studied by scanning electron microscopy (SEM) and energy dispersive X-rays (EDX) and films thickness was measured by using a coulometric method current cathode efficiency was thus calculated (ASTM, 1995).

2 2.1

Experimental details Experimental design

The underlying goal of this study was to correlate corrosion resistance of a chromium coating whit a set of factors such as pH, bath temperature and chromium concentration. Statistical software was used for designing and analyzing the experiments. Response surface methodology was chosen for designing the experiments; this method provides information on direct effects, pair-wise interaction effects and curvilinear variable effects. It also needs fewer combinations than a fully factorial experiment. Central composite design (CCD) of response surface methodology using a three-level factorial design with axial and central points was thus used where the response variable was the corrosion resistance obtained from electrochemical experiments and three factors having three levels: concentration (0.19, 0.38 and 0.57 mol L−1 as Cr(III) ), pH (2.0, 3.0 and 4.0) and temperature (25, 32, 40 o C) as input variable. The order of the experiments was randomized to ensure against the effect of time-related variables

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Suarez et al./ Revista Mexicana de Ingenier´ıa Qu´ımica Vol. 12, No. 1 (2013) 129-141 Table 1. Experimental design showing the set of conditions for chromium plating No

Treatment

Concentration of Cr+3 /(mol L−1 )

pH

Temperature /(◦ C)

Replicates

1 2 3 4 5 6 7 8 9 10 11 12 13

S1P1T2 S3P1T2 S1P3T2 S3P3T2 S1P2T1 S3P2T1 S1P2T3 S3P2T3 S2P1T1 S2P3T1 S2P1T3 S2P3T3 S2P2T2

0.19 0.57 0.19 0.57 0.19 0.57 0.19 0.57 0.38 0.38 0.38 0.38 0.38

2.0 2.0 4.0 4.0 3.0 3.0 3.0 3.0 2.0 4.0 2.0 4.0 3.0

32.5 32.5 32.5 32.5 25 25 40 40 25 25 40 40 32.5

2 2 2 2 2 2 2 2 2 2 2 2 4

Table 2. Composition of chromium baths for chromium plating Concentration /(mol L−1 ) Compound

Solution 1 (S1)

Solution 2 (S2)

Solution 3 (S3)

CrCl3 · 6H2 O H3 BO3 HCOONa CH3 COONa 3H2 O C12 H25 NaO4 S

0.19 0.80 0.095 0.095 3.5 × 10−4

0.38 0.80 0.19 0.19 3.5 × 10−4

0.57 0.80 0.285 0.285 3.5 × 10−4

and also to satisfy independence of observation statistical requirement. Each experiment required two replications and four replications were made for central point, until experiments were completed according to the design layout; adding of central points (as shown in Table 1) led us detecting nonlinearity in responses.

2.2

Electroplating

All chemicals used were reagent grade. Chromium solutions were formulated with chromium chloride as chromium source in the quantities shown in Table 2; sodium formate and acetate were used as complexing agents in equal molar concentration in 1:1 ratio regarding chromium in solution. 50g L−1 boric acid was used as buffer and 0.1 g L−1 sodium lauryl sulfate as wetting agent. Solutions were heated up to 60◦ C for three hours as in (Suarez, 2006). The coatings were deposited on a 2 × 2 cm2 mild steel (AISI 1008) flat sample. All experiments were prepared as follows: firstly, the surface was

mechanically polished using up to 600 grade emery paper (to simulate an industrial condition) and then manual and anodically degreased in sodium hydroxide (5% w/v), after having been activated in sulfuric acid (5% w/v) for 2 minutes and then plated in a nickel Watts bath until a 6 micron thickness was reached. Deionized water was used for rinsing between all steps. Nickel bath composition is listed in Table 3. Conditions during nickel plating were pH 4.5, 55◦ C and 3 A cm−2 cathode current density.

Samples were coated with chromium immediately after nickel plating. Chromium plating was done at constant 8 A dm−2 cathodic current for 2 minutes using graphite as anode. The set of conditions used during chromium plating are presented in Table 1. Nickel and chromium were plated separately but in similar cathodic device. Figure 1 shows the cell used for nickel and chromium plating.

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2.4

Thickness measurement

Chromium film thickness was measured by the coulometric method using the three-electrode cell in a 100 g L−1 sodium sulfate solution (neutral pH) with 0.1 A dm−2 current density, at this conditions potentials close to 1.10V vs SCE were registered on work electrode. The amount of mass dissolved was calculated assuming that the only oxidation reaction occurring on the electrode was (ec 1), this could be concluding by analysis of Pourbaix and predominance zone diagrams (Barrera et al., 2003): Cr → Cr+6 + 6e−

(1)

Average current cathode efficiency was calculated on the other side as the stripped to deposition charge ratio; three points were measured on any sample.

2.5 Fig 1. Nickel and chromium plating cathodic device.

Fig 1. Nickel chromiumofplating cathodic device. Table 3.and Composition Watts bath for nickel plating. Table 3. Composition of Watts bath for nickel plating

Compound −1 Compound NiSO$_4$ Concentration /(g L ) NiCl$_2$ NiSO4 250 H$_2$BO$_3$ NiCl2 60 C$_{12}$H$_{25}$NaO$_4$S H2 BO3 40 (NH$_2$)$_2$CS C12 H25 NaO4 S C$_ H$_ NnaO$_ S$\cdot2$H$_ 1 7$ 4$ 3$ 2$O

(NH2 )2 CS C7 H4 NnaO3 S·2H2 O

2.3

0.25 1

Corrosion measurement

Electrochemical experiments were performed in 100mL volume three-electrode cell with. A 0.2 cm2 area of every sample isolated with an o-ring was used as working electrode. A graphite bar and a saturated calomel electrode (SCE) were used as counter and reference electrode, respectively. Corrosion pattern was evaluated after 1 and 24 hours exposure to neutral 3% NaCl at room temperature (20 ± 1◦ C) using a GAMRY potentiostat. EIS measurements were conducted at open circuit potential (OCP) over a 103 Hz to 5×10−3 Hz frequency range with 10 mV AC amplitude. Potentiodynamic polarization curves were measured between 0.25 V to 0.25 V vs. open circuit potential (OCP) at 0.2 mV s−1 scan rate. 132

Morphology and composition

Composition and surface morphology were examined by EDX analysis and SEM. QUANTA 200 microscope was used with a 20KeV electron beam. Concentration /(g 3 -1}$Results and discussion L$^{ ) 250 603.1 Morphology, thickness and composition 40 1Fig. 2 shows the morphology of representative chromium coatings produced using different operating 0.25 1conditions. Smooth surfaces were observed in all

cases (all samples not shown); however, pH was the parameter mainly affecting surface morphology. When comparing deposits a crack pattern was found for coatings obtained at higher pH values, 3.0 and 4.0 (Fig. 2 c.e.f). while this did not occur for pH 2.0 (Fig. 2 a.b.d). Such effect was seen to be independent of Cr(III) temperature and concentration in the plating bath. Cracks in electrolytic chromium have been explained as being due to incorporating chromium hydride in the coatings which breaks down, thereby causing film cracking (Snyder and Mandic, 2000; Snyder and Jones, 2004). Fig 3, shows chromium coatings thickness measured by coulometric method, chromium atomic percentage measured by EDX and average current cathode efficiency. Good correlation was found between thickness measured by coulometric method and Cr at% obtained by EDX. Data in Fig. 3 shows that thickness was very low for diluted solutions (S1) having 0.19 mol L−1 of Cr+3 , while both thickness and cathode efficiency increased greatly for more

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Suarez et al./ Revista Mexicana de Ingenier´ıa Qu´ımica Vol. 12, No. 1 (2013) 129-141

Fig. 2. SEM micrographs of chromium films plated in different operating condition; (a) S1P1T2 1KX; all others 10KX (b) S2P1T1, (c) S2P2T2, (d) S3P1T2, (e) S3P2T1, (f) S2P3T2.

Fig. micrographs of chromium filmsinplated different operating condition; Fig. 2. 2. SEM SEM micrographs of chromium films plated differentinoperating condition; (a) S1P1T2 1KX;(a) all others S1P1T2 (b) S2P1T1, (c)(f) S2P2T2, 10KX (b) 1KX; S2P1T1,all(c)others S2P2T2,10KX (d) S3P1T2, (e) S3P2T1, S2P3T2. (d) S3P1T2, (e) S3P2T1, (f) S2P3T2. 50 100 *C r*E D X *thic kne s s

0,0

25-°C

2 0,5

1,5

0,0

60

32,5°C 3

40°C

80

40-g /L

30-g /L

100 1,0

4

2,0

0

S 3P 3T 2

S 3P 2T 3

S 3P 2T 1

S 3P 1T 2

S 2P 3T 3

*

S 2P 3T 1

S 2P 2T 2

S 2P 1T 3

S 2P 1T 1

S 1P 3T 2

S 1P 2T 3

S 1P 2T 1

S 1P 1T 2

C r*% *(a tom ic )

0

30

20

c a thode *e ffic ie nc y*(% )

40

0,5

1,5

thic kne s s (m ic rons )

*C r*E D X *thic kne s s

10

40

20-g /L

1,0

2,0

T hic kne s s -/µ m

20

50

60

*

1,5

*

30

80

c a thode *e ffic ie nc y*(% )

C r*% *(a tom ic )

2,5

2,0

thic kne s s (m ic rons )

40

S1

S2

S3

P1

P2

P3

T1

T2

T3

atomic percentage.

*

*

1,0 44.0 Fig203.3.RightRight axis: thickness measured by methodFig Fig plots for thickness measured Fig axis: thickness measured by coulometric and 4. equivalent current BoxBox plots for thickness measured by coulometric method and equivalent current by coulometric method grouped by factor: efficiency; Left axis: EDX analysis of chromium in atomic percentage. concentration (S). pH (P) or temperature (T). 0,5 concentration 10 efficiency; Left axis: EDX analysis of chromium in (S). pH (P) or temperature (T). 20

coulometric metho

S 3P 3T 2

S 3P 2T 3

S 3P 2T 1

S 3P 1T 2

S 2P 3T 3

S 2P 3T 1

S 2P 2T 2

S 2P 1T 3

S 2P 1T 1

S 1P 3T 2

S 1P 2T 3

S 1P 2T 1

S 1P 1T 2

0,0 0 0 concentrated solutions (S2 and S3). Fig. 4 shows pH and temperature caused decreased thickness and box plots of thickness grouped by variable and cathode efficiency. These results were similar to this explained the partial effect of every operating those presented in previous work (Baral and Engelken, condition (concentration. pH and temperature). 2005; He et al., 2007); however, the efficiency values * Average thicker chromium coatings were obtained at obtained were higher compared to those obtained high chromium concentration and variation due to for hexavalent chromium, ranging from 15% to 25% Fig 3. Right axis: thickness measured by coulometric method and equivalent current other parameters was less, while increased solution (Snyder and Mandic, 1974; Snyder and Jones, 2004). efficiency; Left axis: EDX analysis of chromium in atomic percentage.

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133

0,0

20-g

0,5

S1

S2

S3

P1

P2

P3

T1

T2

T3

Fig 4. Box plots for thickness measured by coulometric method grouped by factor: concentration (S). pH (P) or temperature (T). ıa Qu´ımica Vol. 12, No. 1 (2013) 129-141 Suarez et al./ Revista Mexicana de Ingenier´

Fig 5 EDX spectrum for sample plated with S2P1T1 (0.38 mol L-1 Cr(III). pH 2.0. T=25ºC). Fig. 5. EDX spectrum for sample plated with S2P1T1 (0.38 mol L-1 Cr(III). pH 2.0. T=25o C).

Table 4. EDX analysis for chromium coatings at% Sample

Cr

Ni

O

C

S

Fe

Cu

S1P1T2 S1P2T1 S1P2T3 S1P3T2 S2P1T1 S2P1T3 S2P2T2 S2P3T1 S2P3T3 S3P1T2 S3P2T1 S3P2T3 S3P3T2

0.6 0.5 0.6 0.3 31 2.3 33.3 0.8 0 23 39 2.7 2.7

96 97 97 97 34 82.7 55 95.5 97 60.7 50.1 42.2 69.6

– – – – 15 2.1 8.7 – – 4.2 8 18.2 8.9 1

– – – – 15 8.8 – – – 6.8 – 30.5 1.6

– – – – 0.8 0.7 – – – 0.6 0.4 2.1 2.6

3.4 2.5 2.4 2.7 2.2 3.4 3 3.7 3 2.9 2.5 2.4 4.6

– – – – 2 – – – – 1.8 – 1.9 –

Such important influence of concentration coating thickness could have been due to diffusional control during chromium reduction; the effect of pH and temperature were due to the olation reaction between Cr(III) and [OH]− to form [Cr(OH)]−2 , [Cr(OH)]−1 , and [Cr(OH)3 ] to form bridge hydroxyl, which can obstruct Cr(III) from electrode (Baral and Engelken, 2005).

having high chromium quantity. These elements can be co-deposited during plating, as previously observed (Survilliene et al., 2007; Boasong et al., 2006). A copper peak appeared in some coatings; this element was not part of chromium plating solution formulation, meaning that it might have resulted from chromium solution contamination with this metal due to accumulation and drag-in from previous solutions.

EDX spectra for one of the films obtained are shown in Fig. 5. Table 4 shows the composition results for the other coatings. The Elements presents were: iron from substrate, sub-nickel layer nickel and sulfur (sulfur due to the brightener system used) and upper film chromium. Peaks corresponding to carbon and oxygen were also observed for coatings

3.2

134

Potentiodynamic polarization tests

Fig. 6 shows potentiodynamic polarization curves for chromium electrodeposited coatings immersed in 3%

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S3P2T3 S3P3T2

2.7 2.7

42.2 69.6

18.2 8.9

30.5 2.1 c urre2.4 1.9 '2 nt3de ns ity3(A 3c m ) 11.6 2.6 4.6 curves -- for electrodeposited chrom Fig 6. Potentiodynamic

operating conditions in 3% NaCl electrolyte, v=0.2mV s-1.

Suarez et al./ Revista Mexicana de Ingenier´ıa Qu´ımica Vol. 12, No. 1 (2013) 129-141 0,0 S 1P 1T 2

'0,2

S 1P 3T 2

22

'0,4 '0,5 '0,6

S 1P 2T 1

1µ 100n

'0,7

S 2P 2T 2

1

'0,2

10n

100n



10µ

100µ

1m

S 1P 1T 2

c urre nt3de ns ity3(A 3c m ) 0,0 '0,1

E 3vs 3S C E 3(V )

'0,2

S 3P 1T 2

S 3P 3T 2

S 3P 2T 3

S 3P 2T 1

S 3P 1T 2

S 2P 3T 3

1E (3

10m 100m

'2

A

S 3P 3T 2

1n

S 3P 2T 3

100p

S 3P 2T 1

'0,6

0,01

S 3P 1T 2

S 2P 1T 1

S 2P 3T 3

S 2P 3T 1

'0,5

0,1

2

'0,4

S 2P 3T 1

S 2P 1T 3

S 2P 2T 2

'0,3

b)

S 2P 1T 3

S 2P 3T 3

R p/(M Ω /c m )

E 3vs 3S C E 3(V )

'0,1

S 2P 3T 1

0,0

S 2P 2T 2

'2

c urre nt4de ns ity4(A 4c m )

S 2P 1T 3

10m 100m

S 2P 1T 1

1m

S 2P 1T 1

100µ

S 1P 3T 2

10µ

S 1P 3T 2



S 1P 2T 3

100n

S 1P 2T 1

10n

S 1P 2T 3

1n

S 1P 1T 2

100p

S 1P 2T 1

10n

'0,8

'0,7

a)

10µ

'0,3

i+c orr+(A +c m )

E 4vs 4S C E 4(V )

100µ

S 1P 2T 3

'0,1

Fig 77 Polarization curve resultsresults for electrodeposited Fig Polarization curve for electrodeposited chrom chromium coatings; a) corrosion current density (Rp). current density (icorr). b) polarization resistance

S 3P 2T 1

'0,3

(icorr ). b) polarization resistance (R p ). S 3P 3T 2

'0,4

up to the OCP, meaning these coatings perform high polarization resistance in this area. A higher increase S 3P 2T 3 '0,6 in current density with low polarization resistance could be observed due to the beginning of pitting '0,7 .S 1 P 1 T 2 100p 1n 10n 100n 1µ 10µ 100µ 1m 10m 100m 1 ,4 M 2003). (Kaesche, Fig 7 shows corrosion current .S 1 P 2 T 1 '2 c urre nt3de ns ity3(A 3c m ) density (i ) and polarization resistance (R p.S) 1 Pat2 T 3 1 ,2 M corr a) Fig 6. Potentiodynamic Potentiodynamiccurves curves for electrodeposited OCP chromium obtained different of polarization calculated for Tafel at extrapolation .S 1 P 3 T 2 Fig 6. for electrodeposited .S 2 P 1 T 1 operating in 3% NaCl electrolyte, v=0.2mV s-1curves. . 1 ,0 MDifferences close to one order of magnitude chromiumconditions obtained at different operating conditions in .S 2 P 1 T 3 −1 were8 0 found between coatings obtained from dilute to 3% NaCl electrolyte, v=0.2mV s . 0 ,0 k .S 2 P 3 T 1 concentrate solutions. No differences in behavior.Sdue 2P 3T 3 6 0 0 ,0 k to pH and plating temperature were observed. .S 2 P 2 T 2 NaCl. All curves showed active behavior up to OCP .S 3 P 1 T 2 4 0 0 ,0 k 100µ .S 3 P 2 T 1 and no passivation occured. The main difference a) .S 3 P 2 T 3 2 0 0 ,0 k between chromium coatings was in the potential where 3.3 Electrochemical impedance spectroscopy 10µ .S 3 P 3 T 2 pitting began. Current density increases rapidly when 0tests ,0 0 ,0 5 0 0 ,0 k 1 ,0 M 1 ,5 M 2 ,0 M 2 ,5 M 3 ,0 M potential increased in coatings obtained from more 1µ Fig 8 shows spectrums Z .rea in Nyquist diagrams for dilute solution (S1=0.19 mol L−1 as Cr(III)) while, l.( Ω ) electrodeposited chromium, after one and twenty-four for some thicker coatings (thicker chromium films) 100n hours immersed in 3% NaCl. The impedance plots obtained from concentrated solutions (S2=0.38 and 300k showed depressed semicircles which decreased in1Tsize S3=0.57 mol L−1 as Cr(III)), an area of little current ,S 1P 2 10n ,S 1P 2T 1 after 24 h. density increase was observed until close to 100mV '0,5

22

i+c orr+(A +c m )

Z .im .( Ω )

1h

b)

www.rmiq.org 200k Z ,im ,( Ω )

S 3P 3T 2

S 3P 2T 3

S 3P 2T 1

S 3P 1T 2

S 2P 3T 3

S 2P 3T 1

S 2P 2T 2

S 2P 1T 3

S 2P 1T 1

S 1P 3T 2

S 1P 2T 3

S 1P 2T 1

S 1P 1T 2

24h

100k

,S 1P 2T 3 ,S 1P 3T 2 ,S 2P 1T 1 135 ,S 2P 1T 3 ,S 2P 3T 1 ,S 2P 3T 3 ,S 2P 2T 2 ,S 3P 1T 2 ,S 3P 2T 1 ,S 3P 2T 3

current density (icorr). b) polarization resistance (Rp).

0,0

200,0k

400,0k

Z (re a l(( Ω )

Fig 8 Nyquist diagrams for electrodeposited chromium obtained

Suarez et al./ Revista Mexicana de Ingenier´ıa Qu´ ımica Vol. in 12,3% No.NaCl 1 (2013) 129-141 conditions at OCP after: a)1h; b) 24 hours and c)selecte 1 ,4 M 1 ,2 M

h.

1h

.S 1 P 1 T 2 .S 1 P 2 T 1 .S 1 P 2 T 3 .S 1 P 3 T 2 .S 2 P 1 T 1 .S 2 P 1 T 3 .S 2 P 3 T 1 .S 2 P 3 T 3 .S 2 P 2 T 2 .S 3 P 1 T 2 .S 3 P 2 T 1 .S 3 P 2 T 3 .S 3 P 3 T 2

a)

Z .im .( Ω )

1 ,0 M 8 0 0 ,0 k 6 0 0 ,0 k 4 0 0 ,0 k 2 0 0 ,0 k 0 ,0 0 ,0

5 0 0 ,0 k

1 ,0 M

1 ,5 M

2 ,0 M

2 ,5 M

CPE  dl RE

Rs

CPE  f Rc

CPE dl  ,  adl

b) Z ,im ,( Ω )

200k

100k

Ni

CPE f  ,  af

Fe M..O.. M..(OH).. M..(Cl)..

Rc

Rf ,S 1P 1T 2 ,S 1P 2T 1 ,S 1P 2T 3 ,S 1P 3T 2 ,S 2P 1T 1 ,S 2P 1T 3 ,S 2P 3T 1 ,S 2P 3T 3 ,S 2P 2T 2 ,S 3P 1T 2 ,S 3P 2T 1 ,S 3P 2T 3 ,S 3P 3T 2

LZwo

Cl-­‐

RE

24h

 

3 ,0 M

Z .rea l.( Ω ) 300k

Rf

WE

Cr

Zwo

L

Ni

Fe

WE

b) Fig a) 9. a) Equivalent circuit used to fit corrosion Fig 9. a) Equivalent circuit resistance parameters in EIS used tests. to fit b) corrosion physical resistance param physical equivalent. equivalent.

Z (im (( Ω )

layers presented defects (as pores) which were rapidly penetrated by the solution and a small area of steel substrate was constantly exposed to the chloride solution during the corrosion test. 0 0,00 250,00k 500,00k 750,00k The circuit shown in Fig 9, is the usual type of Z ,rea l,( Ω ) circuit used for multi layer coatings (Kaesche, 2003; Tapan; 2007) where Rs represents the circuit’s total c) ohmic resistance, Rc porous resistance and CPEdl (constant phase element) double layer capacitance 200k with correction for the surface roughness and Rf and CPEf, respectively, the film resistance and coating capacitance. Only for thinner films, inductive element (S 1P 1T 2(1h (S 1P 1T 2(24h L appears as can be seen in Fig 8c (parentheses (S 2P 1T 1(1h 100k representation in Fig 9). Inductive element represents (S 2P 1T 1(24h (S 2P 2T 2(1h corrosion products adsorption in electrode surface or (S 2P 2T 2(24h interaction between active and passive areas (Kaesche, (S 3P 3T 2(1h 2003). Table 5 shows parameters of proposed (S 3P 3T 2(24h ((((((((((F IT equivalent circuit. Values for equivalent resistance 0 R f are always at least one order of magnitude greater 0,0 200,0k 400,0k than Rc resistance, this means to R f is the control Z (re a l(( Ω ) resistance in the corrosion process. Diminution Fig 8 Nyquist diagrams for electrodeposited of overall resistance corrosion of the coating after Fig 8 Nyquist diagrams for operating electrodeposited obtained in different operating chromium obtained in different conditions chromium 24 hours then can be attributed to decreasing in in 3% NaCl after: a)1h; after: b) 24 a)1h; hours b) and24 hours conditions in at3%OCP NaCl at OCP and c)selected fitted data 1 and 24 for CPEdl R f . It can be observed to exponents h.c)selected fit data 1 and 24 h. elements are between 0.76-1.00 which means no regular surfaces are presents in all cases, for the CPE  dl These were modeled by using the circuit presented exponents of CPE f it can be seen to values as low as in Fig 9. An inductive effect was also observed in 0.15 are obtained in some cases, this could indicate Rs obtained from low chromium concentration coatings to diffusion phenomena can be occurring in these CPE  fwith short time RE this effect appeared even WE (S1); coatings. Inductive parameter (L) was observed only immersion, indicating Rc that the chromium and nickel for coating obtained from diluted solutions S1 and

Rf

 

LZwo

136

www.rmiq.org CPE dl  ,  adl

Ni

CPE f  ,  af Cl-­‐

RE

Rc

Rf

Zwo

L

WE

Fe M..O.. M..(OH).. M..(Cl)..

Suarez et al./ Revista Mexicana de Ingenier´ıa Qu´ımica Vol. 12, No. 1 (2013) 129-141 Table 5. Parameters for equivalent circuit corrosion for chromium coatings obtained at different operating conditions. Treatment

Time (h)

Rs (Ω cm2 )

S1P1T2

1 24 1 24 1 24 1 24 1 24 1 24 1 24 1 24 1 24 1 12.9 124 24

15.1 14.2 11.8 16.8 15.1 15.2 12.8 13.6 11.3 12.2 13.0 11.9 12.4 13.2 11.6 13.4 14.6 14.0 8.8 2.29E-05 12.3 12.9 12.3

S1P2T1 S1P2T3 S1P3T2 S2P1T1 S2P1T3 S2P2T2 S2P3T1 S2P3T3 S3P1T2 24 S3P2T1

90 80

CPEdl (S*sa cm−2 )

adl

70 pha s e ,(°)

60 50 40 ,S 1P 1T 2 ,S 1P 2T 3 ,S 1P 3T 2 ,S 2P 3T 1 ,S 2P 2T 2 ,S 3P 2T 1 ,S 3P 3T 2

20 10 0

10

#2

10

#1

0

,F IT ,F IT ,F IT ,F IT ,F IT ,F IT ,F IT

,S 1P 2T 1 ,S 2P 1T 1 ,S 2P 1T 3 ,S 2P 3T 3 ,S 3P 1T 2 ,S 3P 2T 3

1

2

10 10 10 F re que nc y,(H z )

1M

CPE f (S*sa cm−2 )

3.40E-05 0.88 4.62E+04 — 3.74E-05 0.87 4.00E+02 5.45E-05 2.84E-05 0.89 4.60E+03 — 4.77E-05 0.84 1.24E+03 9.10E-05 2.76E-05 0.88 9.54E+04 — 2.02E-06 0.75 1.26E+01 3.18E-05 2.49E-05 0.82 8.20E+01 1.03E-05 5.76E-05 0.75 4.20E+02 6.36E-05 2.30E-05 0.86 8.08E+01 1.34E-05 3.97E-06 1.00 7.06E+00 4.66E-05 2.69E-05 0.90 2.10E+03 5.40E-06 2.65E-05 0.87 3.14E+04 1.82E-04 2.77E-05 0.84 6.44E+02 6.30E-05 6.40E-05 0.78 1.63E+01 7.15E-06 5.10E-05 0.82 7.00E+01 4.13E-05 5.60E-05 0.78 8.40E+02 8.74E-06 4.25E-05 0.89 7.70E+01 8.27E-05 7.16E-05 0.83 3.50E+02 1.74E-05 5.70E-06 0.76 6.52E+00 1.29E-05 0.81 3.32E+03 7.95E-05 0.52 3.64E-05 0.86 0.66 1.85E+04 ---2.79E-05 1.45E+04 0.871.17E-043.54E+03 1.02E-06 3.64E-05 0.86 1.45E+04 1.17E-04

2 4 ,h

30

Rc (Ω cm2 )

,F IT ,F IT ,F IT ,F IT ,F IT ,F IT

10

3

10

24)h

4

af

Rf (Ω cm2 )

L(H)

— 0.60 — 0.60 — 0.88 0.74 0.88 0.54 0.67 1.00 0.25 0.84 0.92 0.75 0.82 0.73 0.66 0.93 1.61E+05 0.15 0.66

1.69E+05 1.28E+04 1.43E+04 8.90E+03 3.46E+05 3.14E+04 1.15E+05 6.40E+04 1.66E+05 1.25E+05 7.72E+05 9.20E+04 4.50E+04 2.74E+04 9.50E+05 5.23E+04 6.73E+04 1.12E+04 6.14E+05 — 5.24E+05 1.85E+04

1.55E+07 5.61E+06 1.45E+07 — — — — — — — — — — — — — — — —

hours immersed in 3% NaCl. corresponds to thinner chromium coatings obtained. Fig 10 shows Bode diagrams after twenty-four hours immersed in 3% NaCl. The general behavior observed was the same for all coatings, meaning two time constants; however, one of these decreased rapidly after 24 hours for the less resistant coatings. A large difference of about two orders of magnitude in total impedance was observed between films obtained from diluted to concentrate solutions; chrome surfaces obtained at low pH values presented the highest resistance.

m od)Z )( Ω )

100k

3.4

10k

Corroded surface morphology

Fig 11 shows SEM images obtained for representative coatings after EIS experiments. No pits were observed 100 after 24 hours’ immersion in 3% NaCl; however, the crack pattern which had only been observed for 10 deposits obtained from solutions with pH 3.0 or 4.0, 10 10 10 10 10 10 10 F requenc y)(H z ) was then observed for all coatings, including those obtained at pH 2.0 (Fig 10 a. b). Attack on the cracks’ Fig 10 Bode plotsplots for electrodeposited chromium obtained at different operating conditions. Fig. 10. Bode for electrodeposited chromium edges with an incipient delaminating of the film was after 24 hours immersedoperating in 3% NaCl. obtained at different conditions. after 24 observed for some coatings. 1k

)S 1P 1T 2 )S 1P 2T 3 )S 2P 1T 1 )S 2P 3T 1 )S 2P 2T 2 )S 3P 2T 1 )S 3P 3T 2

#2

)F IT ) )F IT ) )F IT ) )F IT ) )F IT ) )F IT ) )F IT

#1

)S 1P 2T 1 )S 1P 3T 2 )S 2P 1T 3 )S 2P 3T 3 )S 3P 1T 2 )S 3P 2T 3

0

)F IT )F IT )F IT )F IT )F IT )F IT

1

2

3

4

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137

Suarez et al./ Revista Mexicana de Ingenier´ıa Qu´ımica Vol. 12, No. 1 (2013) 129-141

Fig 11.11. SEM SEM micrographs of chromium films corroded after 24 hours in 3% NaCl; (a) S2P1T1. (b) S3P1T1. Fig micrographs of chromium films corroded after(c)24 hours in S3P2T1. (d) S3P3T2. S2P1T1. (b) S3P1T1. (c) S3P2T1. (d) S3P3T2. Table 6. ANOVA (α = 0.05) analysis for Log (icorr ) Factor

Coeff

Dev Coeff

T

Constant -2.349 0.943 -2.49 Concentration -0.054 0.017 -3.25 pH 0.144 0.166 0.87 Temperature -0.016 0.022 -0.74 S = 0.66; R2 = 33.1%; R2 (adjusted) = 24.7%

P 0.020 0.003 0.394 0.466

Table 7. ANOVA (α = 0.05) analysis for Log (R p ) Factor

Coeff

Dev Coeff

T

Constant -2.296 0.699 -3.28 Concentration 0.049 0.012 3.94 corr 0.123 pH -0.104 -0.82 Temperature 0.015 0.016 0.92 2 2 S = 0.49; R = 41.6%; R (adjusted) = 34.3%

Table 6 ANOVA (α=0.05) analysis for Log (i

)

P 0.003 0.001 0.418 0.366

Factor Coeff Dev Coeff T for 1 and 24 hours’0.943 immersion in 3% NaCl.-2.49 EIS test Constant -2.349 3.5 Statistical analysis of results values were obtained from the fixed model presented Concentration -0.054 -3.25 Tables 6 to 9 show ANOVA analysis for i , R and in Fig 9. Rc was 0.017 not analyzed because it was close R derived from potentiodynamic and impedance tests to an order of magnitude less than R and the highest pH 0.144 0.166 0.87 138 Temperature www.rmiq.org -0.016 0.022 -0.74 2 2 S = 0.66 R = 33.1% R (adjusted) = 24.7% corr

p

f

Table 7 ANOVA (α=0.05) analysis for Log (R )

f

P 0.020 0.003 0.394 0.466

Suarez et al./ Revista Mexicana de Ingenier´ıa Qu´ımica Vol. 12, No. 1 (2013) 129-141

Table 8. ANOVA (α = 0.05) analysis for Log (R f 1h ) Factor

Coeff

Dev Coeff

T

Constant -5.055 2.365 -2.13 Concentration 0.292 0.064 4.54 pH 1.727 0.643 2.68 Temperature 0.357 0.002 4.97 Conc x Temp -0.009 0.002 -4.48 pH x Temp -0.063 0.019 -3.24 S = 0.4146 R2 = 65.6% R2 (adjusted) = 57.7%

P 0.044 0.000 0.014 0.000 0.000 0.004

Table 9. ANOVA (α = 0.05) analysis for Log (R f 24h ) Factor

Coeff

Dev Coeff

T

P

Constant Concentration pH Temperature Conc x Temp Conc x pH

4.285 0.063 -1.177 0.137 -0.006 0.040

2.147 0.101 0.404 0.053 0.002 0.019

2.00 0.63 -2.92 2.56 -2.12 2.12

0.059 0.537 0.008 0.018 0.046 0.046

one would that which control the corrosion. In all cases, data was analyzed as Log scale for obtaining normal parameter distribution and met the statistical requirements. The tables show coefficients for the associated regression model with associated standard deviations and also T (T-student distribution) and P values for statistic validation of the hypothesis. ANOVA analysis of icorr and R p (Tables 6 and 7) showed that only chromium concentration in plating solution affected the coatings’ corrosion resistance (less than 0.05 P value); this could have been due (as seen before) to concentration greatly affecting the thickness of the chromium coating obtained. Chromium thickness has been directly related to coating/substrate system corrosion resistance; however, due to pitting caused by chloride ions, no other effects such as porosity or surface effects, were easily detected by this technique. Analysis of R f after 1 and 24 hours’ immersion (Tables 8 and 9) showed that all parameters (concentration, pH, temperature and interaction between some of them) influenced electrodeposited chromium corrosion resistance due to the sensitivity of EIS measurement for detecting differences in complex systems such as coatings/substrate, as in this case [13]. Fig 12 shows contour plots for Log (R f 1h ) obtained from the surface response experiment for coatings obtained from solutions of different chromium (III) concentration regarding pH and temperature during plating. The relationship between different parameters

Fig 12 Contour plots for Log Rf1h for electrodeposited chromium Fig. 12. Contour plots for Log R f 1h for conditions, α=0.05. electrodeposited chromium in different operating conditions, α = 0.05.

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Suarez et al./ Revista Mexicana de Ingenier´ıa Qu´ımica Vol. 12, No. 1 (2013) 129-141 during plating can be seen concerning coatings’ corrosion resistance after 1h in 3% NaCl (R f 1h ). Areas with log Rf1h from 5.7 to 6 had grater than 500k? impedance; these were the more resistant coatings and were only obtained from diluted solutions at high temperatures and low pH values. The operation area where resistant coatings could be obtained were greater for more concentrated solutions and a wide pH and temperature window conditions could be used for S3 (0.57 mol L−1 ) for obtaining major corrosion resistance. The contour profiles also that low pH values provided the most favorable conditions for obtaining high corrosion resistance values (R f 1h ).

Conclusions Electrodeposited chromium coatings were obtained by modifying operating conditions such as concentration, pH and temperature during plating. Differences in morphology were observed, mainly depending on plating bath pH; a crack pattern was observed in high pH conditions. The coulometric method was used for measuring chromium films thickness. This method had good correlation with chromium amount detected by EDX measurements. Difference in chromium coatings thickness was mainly due to chromium solution concentration. Chromium coatings’ electrochemical behavior of was studied by potentiodynamic and EIS experiments in 3% NaCl. The results were analyzed by ANOVA finding that potentiodynamic curves only detected differences due to plating concentration solution greatly affecting coatings thickness while EIS experiments led to seeing all effects and interaction ships between process conditions. Chromium films morphology after the EIS corrosion test showed to that no pitting occurred, but corrosion flaw increased in the edges of cracks thereby initiating coating delaminating. High chromium concentration in solution led to obtaining coatings having good corrosion resistance in a wide window of pH and temperature conditions.

Acknowledgements Division de Investigaci´on Universidad de Colombia, project 202010010654, Macrouniversidades de America Latina y CONACYT projects P45833 and DGAPA and ALFACROM LTDA. 140

Nacional Red de el Caribe, IN102907

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