Keywords: sol-gel, phenylenediamine, electrochemical electric resistance spectrometry, epoxy-amine
Organic-inorganic sol-gel intercrossed coatings are seeing increased involvement as possible replacings for Cr ( VI ) based pretreatments and primers.1-13 The coatings are covalently bonded to the implicit in metal, supplying first-class adhesion.3 Many can be prepared in water-only systems giving low molecular weight intoxicants as the lone by-products. Organic functional groups incorporated into the coatings provide covalent bonding to functional groups in subsequent beds giving a complete coating system covalently bonded to the substrate.6-8 The hydrophobic nature of many of the organic cross-linking agents and tight cross-linking of the metal-oxide colloidal suspension precursors yield first-class barrier belongingss to forestall H2O immersion. Corrosion suppressing agents can be added straight to the coatings or encapsulated in antiphonal microcapsules and incorporated into the coatings.9-11 Inhibiting agents loaded into microcapsules are released as the microcapsules respond to localized additions in pH due to the earliest corrosion when H2O does eventually make the substrate, thereby passivating the metal and forestalling farther corrosion while those freely added travel with incoming H2O.
For this work, siloxane intercrossed organic-inorganic coatings were formulated based on tetramethoxysilane ( TMOS ) and glycidoxypropyltrimethoxysilane ( GPTMS ) . These silanes were foremost reacted to organize inorganic silicon oxide colloidal particles with the epoxide mediety of GPTMS so cross-linked by the familiar epoxy-amine reaction. Previously, aliphatic aminoalkane cross-linking agents were utilised with analysis by electrochemical electric resistance spectrometry ( EIS ) .12-13 This work was extended with two extra aliphatic aminoalkane cross-linking agents, tris ( 2-aminoethyl ) aminoalkane and triethylenetetramine. The current work besides employed the isomers of the aromatic phenylenediamine as the amine cross-linking agents, viz. , o-phenylenediamine, m-phenylenediamine, and p-phenylenediamine. The electrochemical belongingss were examined of these PDA cross-linked coatings which were rather different from those obtained when aliphatic cross-linking agents were used. These coatings besides exhibited much better public presentation as anti-corrosion coatings than those cross-linked by aliphatic aminoalkanes.
Materials. Aluminum alloy 2024T3 substrates were from McMaster-Carr and Q-Panel. Tris ( 2-aminoethyl ) aminoalkane ( TAEA ) , triethylenetetramine ( TETA ) , o-phenylenediamine ( o-PDA ) , and m-phenylenediamine ( m-PDA ) were from Aldrich, p-phenylenediamine ( p-PDA ) was from Fluka. Sol-gel precursors tetramethoxysilane ( TMOS ) and glycidoxypropyltrimethoxysilane ( GPTMS ) were from Aldrich and Alfa Aesar. Diethylenetriamine ( DETA ) was from Alfa Aesar. Noveca„? Fluorosurfactant FC 4432 was from 3M. Acetic acid was from Fisher Scientific, Na chloride ( NaCl ) from VWR, and ammonium sulphate ( ( NH4 ) 2SO4 ) from ICN. All H2O was 18.2 MI ( Millipore ) .
Substrate readying. The aluminium slides were immersed in propanone or methyl alcohol for degreasing, rinsed with H2O, immersed in de-oxidizing solution, rinsed with H2O and allowed to dry in air. Substrates were surface treated by submergence in a dilute solution of GPTMS in 0.05 M acetic acid ( 1:150 v/v ) for 20 proceedingss and allowed to dry in air as antecedently described.14
Coating readying. TMOS ( 0.02 mol ) and GPTMS ( 0.06 mol ) were combined to organize a 1:3 molar ratio mixture of silane monomers. The silane mixture was so added drop-wise to 22 milliliter of acidified H2O ( 0.05 M acetic acid ) with stirring, giving a solution of 15 moles of H2O to one mole of silane monomer. This mixture was covered and aged with uninterrupted stirring for 72 hours to organize colloidal silicon oxide atoms. Following aging, surfactant solution ( 9.4 milliliter, FC 4432, 0.1 % w/w ) and phenylenediamine cross-linking agents ( 0.015 mol in 10 milliliter of methanol/water 1:1 v/v ) , TAEA, TETA, or DETA ( 1:1 GPTMS: amino H plus adequate H2O for equal volumes ) were added to the silane mixture. Cleaned, surface treated substrates were dip-coated in the resulting mixture. Each substrate was immersed three times and allowed to dry in air. The concluding prohibitionist coating thickness is ca. 1.0Aµm.
Word picture techniques. Infrared attenuated entire coefficient of reflection spectrometry was performed on PDA cross-linked sol-gel coated aluminum metal substrates. Measurements were obtained with a Nicolet 380 FT-IR spectrometer on a individual bounciness ATR accoutrement ( Pike Technologies, MIRacle ATR ) equipped with a ZnSe crystal and a standard DTGS room temperature sensor. 150 scans were used to obtain desirable signal to resound ratios for the sol-gel samples, and 32 scans for precursor stuffs. GC-MS was used in concurrence with NMR spectrometry to place byproducts associated with the PDA cross-linking agents. Samples of oxidised PDA were foremost separated utilizing liquid column chromatography, with methyl alcohol as the dissolver and silicon oxide gel as the stationary stage. Aliquots of each set isolated utilizing these techniques were so analysed utilizing an HP 5890 Series II Gas Chromatograph coupled with an HP 5971A Mass Selective-Detector. UV-Vis spectrometry was used to mensurate the changing peak optical density of the oxidising PDA solutions over clip. Experiments were performed on an Ocean Optics USB 2000 Spectrometer, in a 1.4 milliliter, 1 cm way length vitreous silica cuvette.
Coating public presentation. To measure the corrosion suppression abilities of the coatings, aluminum metal substrates cross-linked with PDA isomers, every bit good as a 4th set with no cross-linking agent, were immersed in dilute Harrison ‘s solution. Four taped substrates were coated for each type of gel: three were scribed down to bare metal over a 2.5 centimeter length to measure the coating public presentation at a site of knowing mechanical harm. The 4th was used as a control to mensurate overall corrosion protection. Samples were inspected visually ; good corrosion suppression was judged by a deficiency of aluminium oxide ( white rust ” ) , every bit good as vesiculation, delamination, or staining of the coating.
Electrochemical Impedance Spectroscopy ( EIS ) . The electric resistance measurings were carried out in dilute Harrison ‘s solution ( 0.05 % NaCl and 0.35 % ( NH4 ) 2SO4 in 18 MOhm H2O ) . The electrochemical cell that was used for the EIS experiments consisted of a poly ( vinylchloride ) pipe with an interior diameter of 2.6 centimeters attached to the substrate utilizing Marine GoopA® adhesive. Three cells, supplying replicate consequences, were obtained for each cross-linking agent. A three-electrode constellation was used with the aluminium panel as the working electrode, Pt coated mesh as counter electrode and saturated mercurous chloride electrode ( SCE ) as the mention electrode. Gamryi?’ Instrument PC-4, FAS1, or Reference 600 potentiostats were used to carry on the EIS experiments. A distressing electromotive force of 10 millivolt ( rms ) was applied to the samples. The frequence scope studied was from 100,000 Hz to 0.01 Hz. The open-circuit potency ( OCP ) was used as the bias potency. The measurings started after 100s of OCP monitoring. A 30 minute hold was employed before the start of each back-to-back trial and continued for about 24 hours. The measurings were so taken one time per twenty-four hours for several yearss and thenceforth the figure of yearss to the following measuring bit by bit increased.
The differences seen between the three aliphatic cross-linking agents in the old work12 were obliging adequate to widen that work to include cross-linkers with extra functional aminoalkanes to farther effort to increase cross-link denseness. TETA was chosen so an excess functional aminoalkane ( 6 sum ) was introduced in a additive concatenation to see if a comparable addition in public presentation was obtained such as when DETA ( 5 sum ) was antecedently compared to ethylenediamine ( EDA ) ( 4 sum ) . TAEA ( besides 6 functional aminoalkanes ) was chosen because it has a constitutional ” crosslink point. Structures of all four cross-linkers are shown in Figure 1. Figure 2 compares EIS consequences obtained for DETA, TETA, and TAEA cross-linked samples. TETA showed the worst public presentation of the three. It appears the added flexibleness of the longer ironss more than beginnings any additions imparted by the extra crosslink point. This besides coincides with a difference seen in contact angles of 31.8 ( A±3.2 ) degrees for TETA and 52.1 ( A±2.7 ) for TAEA. The TAEA cross-linked samples performed the best of the three bespeaking the constitutional ” crosslink does so better the crosslink denseness and surfacing public presentation. These consequences, therefore, indicate there is a jurisprudence of decreasing returns trying to increase crosslink denseness through the figure of epoxy-amine reactions with this peculiar sol-gel coating system.
Figure 1. Structures of aliphatic cross-linking agents.
Figure 2. Average electric resistance values for three samples of each aliphatic cross-linked sol-gel coating on AA2024T3.
The three isomers of phenylenediamine besides used as cross-linking agents are shown in Figure 3. Infrared spectrometry ( informations non shown ) of the cross-linked gels displayed extremums consistent with formation of the silicon oxide anchor and epoxy-amine cross-linking antecedently reported.13-14 In peculiar Si-O-Si stretches centered around 1000 cm-1 were seeable, declarative of the siloxane web formation. Evidence of the epoxide ring opening cross-linking reaction can be found in the disappearing of extremums associated with the epoxide ring. For the GPTMS precursor an epoxide ring extremum appears at 3045 cm-1 ( epoxide CH2 antisymmetric stretch ) ; this is the most utile set since it absorbs rather strongly and is non obscured by other extremums in the spectra. This extremum wholly disappeared after the gels were exposed to the PDA cross-linkers and the coatings solidified. The strong scissoring manner of primary aminoalkane attached to aromatic rings typically found at 1638-1602 cm-1 was of low strength or losing wholly bespeaking transition of the primary aminoalkanes to more extensively substituted aminoalkanes.
Figure 3. The three isomers of phenylenediamine used as cross-linking agents: A ) p-PDA ; B ) m-PDA ; C ) o-PDA.
During exposure to DHS, all samples cross-linked with PDA isomers showed colour that was non seen with aliphatic cross-linked samples. The p-PDA cross-linked coatings demonstrated the most important colour, altering from a colorless coating to a clear purple. m-PDA crosslinked samples besides underwent significant colour altering to a green-brown colour whereas o-PDA cross-linked samples changed slightly less significantly to a pale bluish colour. The sample non cross-linked with PDA, like the aliphatics, was colorless, clearly implicating PDA. All three isomers of PDA are susceptible to oxidization, organizing nitroaniline. These merchandises were considered as possible campaigners for the cause of colour within the cross-linked gels. In order to look into the formation of nitroaniline in these systems, UV-Vis spectra of solutions of PDA in methyl alcohol were acquired at the clip of disintegration and once more 24 hours subsequently after standing in air. The resulting spectra are shown in Figure 4. The extremums at 433 nanometers and 450 nanometer for o- and p-PDA, severally, are consistent with the formation of xanthous and yellowish-orange nitroaniline products.15 However, merely really limited colour alteration was observed for m-PDA. This is non consistent with the comparative colour of the PDA cross-linked sol-gels where p-PDA and m-PDA were the most extremely colored with o-PDA demoing merely limited colour. The oxidization merchandise of p-PDA was further characterized by FTIR, GC-MS and 1H-NMR. Infrared spectrometry revealed some new minor soaking up sets in partly oxidized solid samples of p-PDA. These sets appeared at 1547 cm-1, 1326 cm-1, 1126 cm-1, and 857 cm-1, consistent with NO2 antisymmetric stretching, NO2 symmetric stretching, NO2 C-N stretching, and NO2 scissoring manners, severally. A strong set at 1603 cm-1 in
Figure 4: UV-Visible spectra in aqueous solution of ( a ) o-PDA ab initio and after 24 hours, ( B ) m-PDA ab initio and after 24 hours, and ( degree Celsius ) P-PDA ab initio and after 24 hours.
the spectrum of the oxidised solid indicated a di-substituted benzine pealing with two different substituents. These consequences indicate that some of the aminoalkanes are oxidized to nitro groups. This is confirmed by GC-MS and 1H-NMR ( consequences non shown ) . Overall, these consequences faithfully identify the monomeric oxidization merchandise of p-PDA as p-nitroaniline. In drumhead, the oxidative formation of nitroaniline species by the isomers of PDA may happen to a limited grade within the cross-linked sol-gels under caustic conditions. However, their limited production, their uniformly xanthous colour and the mismatch between the ascertained grade of colour of the sol-gels and the comparative responsiveness of isomers of PDA towards nitroaniline formation clearly indicate that they are non the species responsible for the ascertained colour alteration of the PDA cross-linked sol-gels. We attribute the colour to the oxidative polymerisation of the PDA isomers. All three isomers of PDA have been polymerized by mild oxidization ; the colourss seen in these samples are consistent with the colourss reported for the merchandises of oxidative polymerisation of the isomers of PDA.16-18
Samples wholly immersed in DHS were stable and protected the aluminium metal substrates for drawn-out periods of clip. After 22 hebdomads of submergence, gels cross-linked with p- and o-PDA exhibited the best pinhole corrosion protection and Scribe protection, severally. This public presentation is better than that antecedently observed for aliphatic cross-linking agents.12 o-PDA cross-linked gels have the most important pinhole corrosion ; bantam defects in the coating are attacked, nevertheless, small oxidised aluminium is present in these countries. Gels cross-linked with o-PDA do look to protect the Scribe rather good ; really small oxidised aluminium or other marks of corrosion are present in the Scribe after 8 months of uninterrupted submergence. p-PDA cross-linked gels exhibit good Scribe protection, and the fewest pinhole corrosion musca volitanss. However, where pinhole corrosion does occur, it is more terrible than with the other cross-linking agents. m-PDA cross-linked gels exhibit sensible pinhole protection, nevertheless, a considerable sum of oxidised aluminium is noted in the Scribe, bespeaking poorer protection of exposed metal subdivisions. An illustration is shown in Figure 5 comparing p-PDA cross-linked coating with a surfacing composed of uncross-linked coating, a sample in which any cross-linking nowadays is provided by Si-O-Si bonds.
Figure 5. Aluminum metal slides before and after 8 months of dilute Harrison ‘s exposure ( a ) with p-PDA-cross-linked and ( B ) non PDA cross-linked.
EIS consequences for PDA cross-linked coatings yielded much different behaviour than that seen for any of the aliphatic aminoalkane cross-linked samples ; these consequences besides indicate much better overall public presentation than those cross-linked with aliphatic aminoalkanes as the continuance of the submergence clip was for a much longer period. All PDA cross-linked samples showed similar behaviour regardless of isomer ; a representative graph is shown in Figure 6. The samples show capacitive character at high frequences, turning to resistive behaviour at the in-between frequences, and so go capacitive once more at low frequences as demonstrated by the stage angle secret plans. The level resistive part displacements to higher frequences and lower electric resistance values with greater submergence clip. Figure 7 shows the mean electric resistance values obtained for the triplicate samples
Figure 6. EIS consequences for sol-gel sample cross-linked with o-PDA demoing the diminishing resistive level part ; for this peculiar sample, even after 421 yearss, the low frequence part is merely get downing to turn resistive as evidenced by the stage angle secret plan.
of each PDA isomer. It is seen they are instead comparable at earlier submergence times with p-PDA eventually demoing poorer public presentation after many months.
Figure 8 sheathings plots from a surfacing cross-linked with ethylenediamine and the sample shown in Figure 6. It is seen that the aliphatic cross-linked sample begins to demo resistive behaviour at low frequence within a few yearss while that of the PDA sample is comparatively unchanged after 42 yearss ; all aliphatic cross-linked samples showed similar behaviour. This alteration in EIS behaviour and surfacing public presentation is rather dramatic sing the lone difference between the samples is the cross-linking aminoalkane. Similar behaviour to that seen in the PDA cross-linked EIS secret plan has been seen in old probes of sol gel coatings.8,19-23 It has been described as the consequence of two distinguishable beds: a thin, heavy Si-O-Al transition bed formed at the metal interface, shown by the low frequence capacitive part, combined with a thicker, more porous sol-gel bed shown by the high frequence electrical capacity and resistive plateau.21 Few, if any, nevertheless, show this behaviour for such drawn-out periods of clip as the current PDA cross-linked coatings.
To look into for the presence and influence of an interfacial bed, an extra set of samples was prepared that did non hold the GPTMS pre-treatment ; that pre-treatment would be the most likely beginning
Figure 7. Average electric resistance values for three samples of aluminium metal coated with sol-gels cross-linked with each PDA isomer.
Figure 8. Bode graph illustrations for 1,2-PDA and EDA cross-linked coatings.
for an interfacial bed with the present coating system. In the chief colloidal suspension, most of the Si-O-H medieties should be bound up to the interior of the colloidal particles to organize the atoms ; there should still be plenty to supply first-class adhesion to the substrate by itself, but with the absence of the pre-treatment, the denseness of the Si-O-Si bonds should be greatly reduced. Some grounds for this was seen in Figure 5 ; when an amine cross-linking agent was n’t used, the coating performed really ill, bespeaking decreased sums of Si-O-Si cross-linking. With an amine cross-linking agent, the infinite between the atoms should be filled and cross-linked by either the aliphatic or PDA cross-linkers. Figure 9 shows a representative Bode secret plan of the samples without pre-treatment. While the resistive part is slightly lower than when the GPTMS pre-treatment was present, neither the overall electric resistance nor the secret plan form appear to alter dramatically, at least non at earlier exposure clip. It does get down to demo resistive behaviour at low frequences much sooner than when the pre-treatment was used but this would most likely be due to a stabilising consequence from the PDA aminoalkanes responding to the epoxides of the pre-treatment. This suggests the behaviour is due to something within the surfacing itself, for this behaviour was non seen with any of the aliphatic cross-linking agents, thereby implicating the PDA isomers since that is the lone difference between the coatings.
Figure 9. Bode graphs of 1,4-PDA cross-linked sol-gels, one with GPTMS pretreatment on the aluminium metal substrate, one without.
There are several possible accounts for the enhanced lastingness of the PDA cross-linked coatings. One would be the presence of a forepart ” where the cross-linkers are more extensively polymerized, tantamount to the incursion of the oxygen-containing electrolyte ; as noted earlier, we attributed the colour seen to the oxidative polymerisation of the PDA cross-linkers. Nearer the substrate, the PDA has non polymerized, or is less extensively polymerized, such that a capacitive-only response is measured for the lower part of the coating. The on-going polymerisation of the PDA cross-linkers could function as an O scavenging mechanism, forestalling it from perforating through to the surface ; conductive polymers have been shown to scavenge oxygen.24 Figure 10 depicts this scenario, along with a diagram of the circuit theoretical account used to suit the information at earlier exposure clip ; the theoretical account fits the informations with all PDA isomers.
Figure 10. Schematic of surfacing with a capacitive part as modeled by CPE2 and a part that has localized conductive parts caused by the polymerisation of PDA.
The 2nd changeless stage component ( CPE ) shown in Figure 10 can besides be placed in series behind the 2nd opposition, therefore parallel to the first CPE, and that theoretical account fits the informations every bit good and outputs about indistinguishable values for the assorted elements. This suggests a transmittal line theoretical account may besides be appropriate ; a transmittal line theoretical account is normally used with conductive polymers in which the charge transportation through the coating is through both ionic conductivity in the pores and electronic conductivity through the polymer.25 When polymerized, phenylenediamine outputs poly ( aminobenzine ) -like constructions that are conductive, hence, the PDA trapped in the interstitial infinites between the silicon oxide atoms may be capable of go throughing current and supplying a 2nd conductive tract with separate electrical capacity. Conductive polymers of poly ( aminobenzine ) , poly ( pyrrole ) , and related species have been used to supply corrosion protection as surface interventions and in intercrossed surfacing systems.26-27 Poly-PDA movies have been examined for corrosion suppression on steel.28-29 The PDA oligomers formed by oxidative polymerisation of the cross-linkers used in this survey could be expected to work in a similar mode, supplying electrochemical corrosion suppression. Attempts to happen conduction with 4-point investigation and conductive AFM were unsuccessful but it should be noted that poly-PDA is usually merely semi-conductive in nature ; combined with the silicon oxide atoms, the conduction may merely be excessively low for measuring.
Finally, the EIS behaviour seen could so be from an interfacial bed, merely non aluminum oxide or a Si-O-Si bed. If the low frequence electrical capacity is from the aluminium oxide bed, it should be seen when aliphatic cross-linking agents are used and those samples should besides last as long ; neither was the instance. If it is due to a Si-O-Si interfacial bed, it should besides be seeable with those samples because the lone alteration was the cross-linking agent and, once more, it should last as long. The denseness of epoxy-amine links between the pre-treatment epoxides and PDA aminoalkanes may be far greater than with aliphatic cross-linkers, with less C concatenation at the immediate interface, moving to better stabilise the bed and increasing surfacing length of service ; the level aromatic rings could be expected to pack together more tightly. However, that does non explicate the improved corrosion public presentation seen when a abrasion is introduced through the coating ; the bare aluminium in the abrasion should eat merely every bit rapidly as the metal in a abrasion with any other coating.
Aluminum metal 2024T3 has been coated with an organic-inorganic intercrossed sol-gel coating with epoxide functionalized silica atoms cross-linked by the isomers of phenylenediamine. These coatings demonstrated important betterment in corrosion protection when compared to tantamount coatings in which the lone difference was the amine cross-linking agents were aliphatic in nature. The coatings were studied with electrochemical electric resistance spectrometry which showed immensely different behaviour as compared to aliphatic amine cross-linked coatings. The unmoved oxidative oligomerization of the PDA isomers with possible conduction has been identified as the likely beginning of the improved public presentation and electric resistance behaviour.