Delamination Rates Research Articles

Bonding performance optimization of homogeneous and hybrid hardwood CLT using the Taguchi experimental design method

The favorable properties of the available hardwood resources in Appalachia make them predestined for various high-value-added products. Between them we find mass timber products also, however, the utilization of hardwood species for such construction products comes with several new challenges. Among these, one of the most crucial issues to overcome is ensuring adequate bonding strength and durability. This research aims to optimize the bonding performance of red oak (RO) and red oak-yellow poplar (RO-YP) hybrid Cross-Laminated Timber (CLT), utilizing a one-component polyurethane (1-C PUR) adhesive an surface pretreatment with a primer. For optimization, the Taguchi method was utilized to quantify the effect of four factors, the primer ‘s concentration, adhesive spread rate, clamping pressure buildup time, and clamping pressure on delamination and shear strength across various layup combinations. The focus predominantly rests on mitigating delamination, as the analysis revealed that shear strength and wood failure manifested no notable variance across the investigated settings. Optimal manufacturing parameters’ setup values were identified, significantly diminishing the delamination rates below established thresholds for both YP and RO-YP hybrid panels. The homogenous RO CLT specimens alone remained above the desired level after optimization. The research concludes by outlining specific optimal settings for each wood combination, offering valuable insights for improving CLT bonding efficacy. These findings not only enhance the understanding of adhesive behaviors in hardwood CLTs but also propose practical guidelines for manufacturing, potentially aiding in the broader adoption and application of such materials in construction.

Augmenting bonding properties of OSB and Chinese fir hybrid cross-laminated timber by sandblasting pretreatment

The investigation was conducted into the adhesive performance within hybrid cross-laminated timber (HCLT) composed of sandblasted Chinese fir and oriented strand board (OSB) which was originally covered by cured adhesive during the manufacture leading to very fragile bonding. HCLT specimens with three layers were bonded utilizing three structural adhesives which were polyresorcinol formaldehyde (PRF), polyurethane (PUR), and polyisocyanate (EPI). Surface roughness and wettability testing ascertained that sandblasting could effectively change the original resin layer and microtopography on the surface of OSB laminates, significantly modified the level of roughness on the surface, thereby providing a basis for vertical spreading and permeating of the adhesives. Sandblasting pretreatment consistently augmented the dry bonding shear strength about all the three adhesives, achieving the most substantial increases of 53 %, 39 %, and 37 % for PRF, PUR, and EPI, respectively. The pretreatment further reducing delamination rates to about a half in cyclic delamination tests of all three adhesives. Fluorescence microscopy detection of the bonding interfaces revealed a significant increase in adhesive penetration depth which had been further confirmed by the bonding interface thickness measurement. Surface roughness generally had a negative effect on dry shear bonding strength, especially with EPI adhesive, while surface wettability was positively correlated with bonding strength and adhesive layer thickness. This study introduced an environmental and efficient method to enhance the bonding performance of HCLT.

Effects of secondary bonding process parameters on the bonding and mechanical properties of bamboo scrimber composite: Glue type, spread rate and pressure

Bamboo scrimber composite (BSC), which demonstrates huge potential in construction applications is a novel composite material prepared from bamboo and phenolic resin. The aim of this study was to address the issue regarding the size of BSCs via a secondary bonding mechanism. In this work, the effects of glue type, spread rate and pressure on the bonding and mechanical properties of secondary bonding-bamboo scrimber composites (S-BSCs) were investigated. The bonding properties were tested through shear and delamination tests, and the mechanical properties were characterized by modulus of rupture (MOR), modulus of elasticity (MOE) and compressive strength(CS). The results of the study revealed that the bonding properties of S-BSCs were optimized when Phenol-Resorcinol-Formaldehyde resin (PRF) was used as the glue, at a spread rate and pressure of 275–300 g/cm2 and 1.2 MPa, respectively. The highest shear strength and wood failure percentage, as well as the lowest boiling delamination rates for two cycles were achieved at a PRF spread rate of around 275–300 g/cm2, under a pressure of 1.0 MPa. which were 24.84 MPa, 90.19%, 0% and 0.77% respectively. On the other hand, the lowest boiling delamination rates of 0.23% and 0.89% for two cycles, respectively were found at a PRF spread rate of 225–250 g/cm2 under a pressure of 1.2 MPa. The study revealed that the values of the MOR and MOE, as well as of CS for the S-BSCs were comparable with those of the BSC samples (146.98 MPa, 14.92 GPa, 125.49 MPa) in the pressure ranges of 144.73–148.10 MPa, 13.77–15.05 GPa, and 120.61–125.86 MPa, respectively. Also, it was found that the mechanical properties of S-BSCs were independent of the type of glue, spread rate, and pressure. Overall, this study determined the effect of secondary bonding process on the bonding and mechanical properties of BSCs.

Corrosion Protection of Galvanised Steel Using Organic Coatings Containing Inhibitive Additives Based on Benzothiazoyl Succinic Acid

It is increasingly apparent that the replacement of highly effective, but environmentally unacceptable chromium (VI)- based inhibitive pigment technologies within protective coatings requires the use of combinations of anti-corrosion additives. There is currently significant interest in using organic inhibitors containing hetero-atoms of nitrogen and/or sulphur in conjunction with conventional Cr(vi)-free inorganic pigments as a means of providing equivalent corrosion protection. One such organic inhibitor is benzothiazoyl succinic acid (BTSA), which is presently commercially available as an in-coating anti-corrosion additive, which can improve the protection capability in existing chromate free formulations. In this work we investigate the ability of BTSA to mitigate corrosion-driven organic coating failure within model coatings applied to hot-dip galvanised steel. In addition, the ability of BTSA to inhibit localised corrosion on bare zinc surfaces when released from a protective coating into corrosive electrolyte within a penetrative defect is also evaluated. A combination of time-lapse imaging and in-situ scanning Kelvin probe (SKP) potentiometry is employed to quantify cathodic delamination rates of model poly-vinyl butyral (PVB) coatings containing varying levels of BTSA additions. It is demonstrated that in-coating BTSA, both added directly to PVB or stored as an exchangeable anion in a layered double hydroxide “smart-release” pigment reduce rates of cathodic disbondment by up to a factor of 20. For both methods of incorporating in-coating BTSA, an increasing loading produces a progressive decrease in delamination rates and a transition from linear to parabolic coating failure kinetics. In addition, it is also demonstrated that BTSA dissolved in the aqueous NaCl (aq) electrolyte applied to a defect can also significantly reduce the rate of cathodic disbondment observed for an unpigmented PVB coating. Highly effective inhibition of localised corrosion of bare HDG surfaces by BTSA additions made to NaCl (aq) electrolyte is demonstrated by a combination on SVET and potentiodynamic studies. Linear polarisation resistance experiments demonstrate an inhibition efficiency of 95% for a 10 mM BTSA addition made to 1% w/v NaCl (aq) at pH 7, while SVET-derived, area averaged current density value are shown to decrease by over an order of magnitude compared with an un-inhibited case over a 6 h immersion period. Using polarisation curves obtained for HDG surfaces in the presence and absence of varying concentrations of dissolved BTSA, a mixed inhibition mechanism is proposed. At intermediate concentrations (1 – 10 mM), BTSA strongly adsorbs on the oxide covered Zn surface via its two carboxylate functional groups, thus hindering electron transfer reactions and providing net cathodic inhibition. At higher concentrations (> 10 mM) an additional reinforcement of any weak points in the adsorbed layer is achieved by virtue of the formation of an insoluble salt formed by reaction with Zn(ii) cations, producing a positive shift in free corrosion potential. In-coating BTSA appears to stifle cathodic disbondment by adsorbing strongly on the intact organic-coated metal, which in turn hinders underfilm oxygen reduction and the dissolution of the amphoteric zinc(hydr)oxide layer at the metal-polymer interface.

Dynamic performance of industrial robots in the secondary carbon fiber-reinforced plastics machining

Carbon fiber-reinforced plastics (CFRPs) find many applications given their superior properties. These materials are usually formed using a near-net-shape method that requires secondary machining, such as drilling and trimming, after molding. Industrial robots are becoming increasingly popular machining tools in industries exhibiting high demand for CFRPs. However, it remains challenging to achieve high dimensional accuracy when using such robots and dynamic performance is poor. We experimentally investigated the dynamic properties of the tool tip according to the dominant robot posture during CFRP secondary machining. Based on the results, multi-layer perceptron models were developed to predict the dominant natural frequency and dynamic stiffness of the tool tip. The minimum and maximum mean absolute percentage errors were 1.99 and 7.94, respectively; the error changed markedly with robot posture. Our models improved CFRP robotic machinability. Experimentally, the delamination rates of drilled holes decreased by 15 % and 75 % in terms of length and area, respectively, and the trimmed surface roughness improved by 27 %.

Molybdate and Phosphate Cross-Linked Chitosan Films for Corrosion Protection of Hot-Dip Galvanized Steel.

Environmentally friendly and sustainable methods to protect hot-dip galvanized (HDG) steel from corrosion are extensively studied. Films of the biopolymer polyelectrolyte chitosan were ionically cross-linked in this work with the well-known corrosion inhibitors phosphate and molybdate. Layers on this basis are presented as components in a protective system and could, e.g., be applied in pretreatments similar to a conversion coating. For the preparation of the chitosan-based films, a procedure involving sol-gel chemistry and wet-wet application was utilized. Homogeneous films of few micrometers thickness were obtained on HDG steel substrates after thermal curing. Properties of chitosan-molybdate and chitosan-phosphate films were compared with purely passive epoxysilane-cross-linked chitosan, and pure chitosan. Delamination behavior of a poly(vinyl butyral) (PVB) weak model top coating studied by scanning Kelvin probe (SKP) showed an almost linear time dependence over >10 h on all systems. Delamination rates were 0.28 mm h-1 (chitosan-molybdate) and 0.19 mm h-1 (chitosan-phosphate), ca. 5% of a non-cross-linked chitosan reference and slightly higher than of the epoxsyilane cross-linked chitosan. Immersion of the treated zinc samples over 40 h in 5% NaCl solution yielded a 5-fold increase of the resistance in the chitosan-molybdate system, as evidenced by electrochemical impedance spectroscopy (EIS). Ion exchange of electrolyte anions with molybdate and phosphate triggers corrosion inhibition, presumably by reaction with the HDG surface as well described in the literature for these inhibitors. Thus, such surface treatments have potential for application, e.g., in temporary corrosion protection.

The effect of laser surface textures on the adhesion strength and corrosion protection of organic coatings - Experimental assessment using the pull-off test and the shaft load blister test

Adhesion is necessary for an effective corrosion protection of metallic surfaces through organic coatings. Even the best anticorrosive coatings fail if it is not well adhered to the substrate. Laser surface texturing (LST) process has been widely used for enhancing adhesion between coatings and steel surfaces in various applications. In addition, recent researches have proven that LST is an effective option for pretreatment of steel surfaces prior to organic coating deposition. This work reports an investigation on the adhesion strength of organic coatings deposited onto AISI-A36 carbon steel laser-textured surfaces. Nanosecond LST via the direct laser writing (DLW) method was used to fabricate V-shaped grooves on the steel surfaces. The distance between consecutive grooves was varied to obtain different surface topographies with controlled surface textured area (TA) ranging from 10 % to 60 %. Conventional bead-blasted surfaces were used as a reference. Surface morphology and topography were characterized through roughness and tortuosity. The frequently used pull-off test (ISO 4624) and a shaft load blister test (SLBT) were used to determine the adhesion of the epoxy coating. In addition, accelerated corrosion tests were carried out to assess the effects of the laser textured surface conditions on the corrosion propagation. After running the pull-off test, all the surfaces' conditions exceeded the minimum pull-off strength value of 5 MPa stated in ISO 12944-9. The results for the SLBT showed that the adhesive strength is highly influenced by the laser-textured area. The geometrical aspects of the groove structure promote phenomena such as mechanical interlock and mechanical hooking of the coating, which affect the adhesion failure mechanism of the organic coating during the test. For lower TA, the crack propagation occurs mainly at the coating/steel interface. However, increasing the TA shows that the crack propagation is stopped near the V-shaped grooves due to the complex shape and must deviate inside the coating for further propagation. Finally, adhesion strength results were correlated with coating delamination rates and an indirect relationship was found.

Role of Smart-Release Pigments in Preventing Corrosion Driven Cathodic Disbondment of Organically Coated Hot Dip Galvanised Steel

The role of smart-release corrosion inhibitive pigments in preventing cathodic delamination of organically coated hot-dip galvanized steel (HDG) is investigated. The pigments consisted of hydrotalcite (HT) exchanged with a range of inorganic and organic anionic species and were dispersed in a model PVB coating. A scanning Kelvin probe (SKP) technique was used to determine cathodic delamination rates, and the inhibition efficiencies obtained for inorganic ions increased in the order < < < < < < The inhibition efficiencies for organic-based pigments increased in the order triazole <phenylphosphonate <trans-cinnamate <benzoate <salicylate <benzotriazole. The inhibition efficiency afforded by the best performing organic inhibitor, benzotriazole (BTA), rivalled that of HT containing stored chromate anions. Findings are consistent with HT-BTA acting to sequester anions from the underfilm electrolyte, releasing BTA− which subsequently strongly adsorbs on the underfilm metal surface but can also form an insoluble Zn-BTA precipitate at the coating-defect boundary.

Delamination of the Annulus Fibrosus of the Intervertebral Disc: Using a Bovine Tail Model to Examine Effect of Separation Rate.

The intervertebral disc (IVD) is a complex structure, and recent evidence suggests that separations or delamination between layers of the annulus may contribute to degeneration development, a common cause of low back pain The purpose of the present experiment was to quantify the mechanical response of the layer-adjoining interlamellar matrix at different rates of separation. Understanding the rate-dependency of the interlamellar matrix, or the adhesion between adjacent layers of the disc, is important as the spine experiences various loading velocities during activities of daily living. Twelve discs were dissected from four bovine tails (three extracts per tail). Two multi-layered annulus samples were collected from each IVD (total = 24, mean bond width = 3.82 ± 0.96 mm) and randomly assigned to a 180° peel test at one of three delamination rates; 0.05 mm/s, 0.5 mm/s, or 5 mm/s. Annulus extracts were found to have similar maximal adhesion strengths (p = 0.39) and stiffness (p = 0.97) across all rate conditions. However, a significant difference in lamellar adhesion strength variability was observed between the 5 mm/s condition (0.96 N/mm ± 0.31) when compared to the 0.5 mm/s (0.50 N/mm ± 0.19) and 0.05 mm/s (0.37 N/mm ± 0.13) conditions (p < 0.05). Increased variability may be indicative of non-uniform strength due to inconsistent adhesion throughout the interlamellar matrix, which is exacerbated by increased rates of loading. The observed non-uniform strength could possibly lead to a scenario more favourable to the development of microtrauma, and eventual delamination.

The effect of pulse current cathodic protection on cathodic disbondment of epoxy coatings

For coated pipelines with cathodic protection, cathodic delamination has been judged as the main reason of coating delamination. This work aims to use the pulse current cathodic protection (PCCP) to mitigate the coating cathodic delamination. Cathodic delamination tests, electrochemical impedance spectroscopy and wire beam electrode method (WBE) were used to study the effect of PCCP on the mitigation of coating disbondment. The results show that PCCP could successfully retard the cathodic delamination rates of organic coatings. Under different cathodic protection potential, the delamination area of the coating protected using PCCP is much smaller than that using direct current cathodic protection. With the increasing pulse frequency and duty ratio, the mitigation effect decreases. Within the range of pulse frequency from 5 Hz to 500 Hz and duty ratio from 0.1 to 0.9, the results of cathodic delamination tests and WBE tests indicate that when the cathodic potential is −1.5 V, the mitigation effect reaches the maximum with the pulse frequency of 5 Hz and the duty ratio of 0.1. PCCP alleviates cathodic delamination of coating mainly by intermittently shifting the surface polarization potential to positive direction and reducing the consumption of cathodic reaction charge.

The contribution of Zn(II) and phosphate anions to the inhibition of organic coating cathodic disbondment on galvanised steel by zinc phosphate pigment

In-situ scanning Kelvin probe (SKP) measurements are used to investigate the relative contributions of in-coating zinc (II) cations and phosphate anions to the inhibition of corrosion-driven cathodic disbondment by zinc phosphate (ZnPhos) pigments. Using hot-dip galvanized steel (HDG) substrates, delamination rates of model polyvinylbutyral (PVB) coatings comprising different pigment volume fractions (ϕ) of phosphate-loaded hydrotalcite and Zn(II)-exchanged bentonite are compared with those established in the presence of ZnPhos. The most powerful inhibitory effect is obtained using in-coating Zn2+, while ZnPhos pigments inhibit cathodic disbondment rather weakly and as such the principal function of phosphate is to control Zn2+ solubility.

Bonding performance of adhesive systems for cross-laminated timber treated with micronized copper azole type C (MCA-C)

The feasibility of manufacturing cross-laminated timber (CLT) from southern yellow pine (United States grown) treated with micronized copper azole type C (MCA-C) preservative was evaluated. Lumber (2x6 visually graded no. 2 boards) was treated to two retention levels (1.0 and 2.4 kg/m3), planed to a thickness of 35 mm, and assembled along with an untreated control group using three adhesive systems following product specifications: melamine formaldehyde (MF), resorcinol formaldehyde (RF), and one-component polyurethane (PUR). Block shear and delamination tests were conducted to examine the bonding performance in accordance with ASTM D905 and ASTM D2559 Standards, respectively. One-way analysis of variance and Kruskal-Wallis H test were conducted to evaluate the effects of preservative retention and adhesive type on block shear strength (BSS) and wood failure percentage (WFP). Regardless of adhesive type, the 1.0 kg/m3 retention treatment significantly lowered BSS compared to the untreated control. CLT composed of the laminations treated at 2.4 kg/m3 maintained BSS when PUR and RF were used but not MF. The average WFP of each CLT configuration ranged from 89% to 99%. The untreated CLT specimens did not experience any delamination under accelerated weathering cycles. The delamination rates of the treated specimens assembled using MF and RF increased with the preservative retention level, while PUR provided delamination rates less than 1% to the laminations treated at both levels. These combined data suggest that, under the conditions tested, PUR provided overall better bonding performance than MF and RF for MCA-C treated wood.

Smart-release inhibition of corrosion driven organic coating failure on zinc by cationic benzotriazole based pigments

A novel cationic benzotriazole pigment (CBP) based on the benzotriazolium cation (BTAH2+) exchanged into a sulfonated organic resin has been synthesized and evaluated as a means of inhibiting the corrosion-driven cathodic disbondment of organic coatings from the surface of galvanized steel. The CBP is acidic in nature (BTAH2+ pKa ≈ 1.1) and is intended to be compatible with acidic coating formulations such as etch-primers. Delamination rates, as measured using a scanning Kelvin probe (SKP), were found to decrease monotonically with increasing CBP volume fraction (ΦCBP) and to approach zero when ΦCBP = 0.1. The mechanism of CBP operation is described.

Development of a nanostructured Ce(III)-Pr(III) film for excellently corrosion resistance improvement of epoxy/polyamide coating on carbon steel

Chemical treatment of carbon steel (CS) specimens was done by an eco-friendly Ce (III)-Pr(III) composite nanofilm. The impact of composite film deposition on CS on the epoxy coating (EP) capability for reduction of CS corrosion and delamination rates was examined. The Ce (III)-Pr (III) thin film morphology was studied by field emission-scanning electron microscopy (FE-SEM) and energy dispersive spectroscopy (EDS) methods. The EP anti-corrosion capacity promotion on CS by deposition of Ce(III)-Pr(III) composite film was investigated by electrochemical impedance spectroscopy (EIS) and salt spray test. Cathodic delamination (CD) rate of EP/Ce (III)-Pr (III) system was also examined. Furthermore, molecular dynamics (MD) simulations were applied to get microscopic-level insights into the aminoamide hardener/epoxy coating interaction mechanism with Ce (III)-Pr (III) film. It was shown that the film uniformly deposited is mainly composed of oxide/hydroxide of cerium (III) and praseodymium (III) and without crack and defect. Results revealed that the Ce (III)-Pr (III) composite thin film significantly promoted the EP anti-corrosion capability and decreased the CD rate. The computational results proved that the curing agent and crosslinked EP resin adsorbed to praseodymium and cerium oxide surfaces mostly through electrostatic interactions of N and O heteroatoms with surface metal atoms.

Understanding the influence of water hydrogen bonding on the cathodic delamination rate of coated steel substrates from pre-exposure characterization

Establishing anticorrosive properties of new and modified coating formulations is a time intensive and expensive process. It is therefore desirable to develop methods capable of predicting corrosion performance of new or modified coating formulations from ambient measurements. Our objective was to understand how ambient, pre-exposure water hydrogen bonding affects cathodic delamination. Nano-carbon allotropes were incorporated into a parent epoxy-amine matrix material applied to steel substrates to alter water hydrogen bonding interactions. We validated that water hydrogen bonding distributions at the air/polymer interface were directly related to measured cathodic delamination rates and were useful in predicting experimental cathodic delamination rates.

Comparison of fatigue in fiber-backed PVDF and PFA fluoropolymer linings

In this work comparative mechanical fatigue experiments were performed in order to quantify the mechanical stress delamination rates for both PVDF and PFA lining materials. Evidence was found for a Paris Law behavior when samples are cycled in blister test configurations. PFA liners exhibited crack growth constants C=0.0486cm/cycle and n=0.9 while PVDF liners exhibited crack growth constants C=0.0999cm/cycle and n=0.8. Linear crack growth rates were observed which ranged from 0.042 ((a/a0)/cycle) at 3.10 bar up to 1.47 ((a/a0)/cycle) at 4.48 bar for PVDF and 0.024 ((a/a0)/cycle) at 2.59 bar up to 0.262 ((a/a0)/cycle) at 3.79 bar for PFA. PFA liners were found to fail at 5.52 bar while PVDF liners failed more violently at 6.21 bar. Overall fatigue ratings of the PVDF vs PFA linings should balance the faster delamination rates of PVDF liners vs the lower strength of PFA liners. It is unlikely that vessels with PVDF or PFA liners under fatigue failure due to vacuum-induced mechanical stresses since much larger stresses were required to cause low cycle fatigue failure. Instead, sample delamination is likely due to thermal stresses arising from a mismatch in thermal expansion between liner, fiber backing, epoxy, and metal substrate.

Cathodic driven coating delamination suppressed by inhibition of cation migration along Zn|polymer interface in atmospheric CO2

The degradation of the Zn|polymer interface is inhibited by CO2 gas in a humid environment. The inhibition mechanism varies greatly for different polymer matrices and depends on the affinity of the polymer to CO2. Coatings based on polymers with high affinity to CO2 such as polyacrylamide show high delamination rates due to the fast uptake of water. In this case, the cation transport that causes the initial pull down of potential for initiating the oxygen reduction reaction occurs via the polymer. Here CO2 decreases water uptake due to competitive absorption into the polymer matrix, inhibiting the delamination rate. CO2 can quickly reach the interface of polymers with functional groups with a low affinity to water and CO2, such as polyvinyl butyral and polyvinyl alcohol. In this case, the inhibition of the delamination rate is achieved by a strong decrease in cation migration rate at the Zn|polymer interface accompanied by the formation of mixed hydrozincite/absorbed CO2 layers on the ZnO surface underneath the polymers. Further experiments showed that the presence of CO2 accelerates anion migration, suggesting an influence of CO2 on the surface charge at the Zn|coating interface, thus affecting ion migration. Inhibition of cation migration has never been reported before and should be taken into account into the mechanism of cathodic-driven delamination on Zn under atmospheric conditions.

The effect of delamination size and location to buckling behavior of composite materials

In this study, the size and location of delamination effects to layered composites' mechanical properties in buckling behavior are inspected experimentally. Composite plates were produced in accordance with the different dimensions of delaminations and the positions between the layers. These plates consist in delamination rates of a/L=0.3,0.5,0.7 (a: Delamination width and L: Buckling length). The specimens were subjected to buckling tests with placed on its two edges, while the others are free. In the experiments two results are considered. First, we determined load-displacement graphs. Critical buckling loads (Pcr) were determined from the obtained load-displacement graphs. Second, we acquire lateral displacements, which occurred with the effect of applied load in the delamination area in direction of plate thickness. By comparing Pcr with each other, an optimum delamination dimension was determined for composite materials with delamination zones. In the applications, the negative effect on the mechanical properties due to delaminations can be seen, proportionally according to the delamination location.

Influence of Uniaxial Deformation on Surface Morphology and Corrosion Performance of Trivalent Chromium Based Coatings for Packaging Steels

The packaging steel industry has many products for the consumer market which are deformed to a required shape. Products produced in this way undergo different types of deformation, including uniaxial, biaxial and plane strain. In a situation where the steel is coated and then deformed, the coating must still offer a substantial level of protection for inhibiting corrosion. An In Situ scanning kelvin probe (SKP) experiment was used to investigate the kinetics of organic coating disbondment on uniaxially deformed trivalent chromium coated steel. The studied material was a model trivalent chromium coated steel prototype, designed for over coatings with a lacquer or polymer laminate. The samples were uniaxially deformed to 5%, 10%, 20%, 25% strain using a Hounsfield/Tinius Olsen tensile tester prior to any further experimentation. The strain percentages selected corresponded to the yield point elongation zone, strain-hardening zone and early and late necking zones respectably. A model Poly(vinyl) butyral (PVB) coating was applied to the uniaxially deformed samples. A defect was created in the PVB coating and the sample exposed to aqueous sodium chloride electrolyte in 95% relative humidity. The disbondment kinetics of the polymer overcoat were measured every hour using the SKP, for up to 120 hours. The disbondment kinetics have been quantified as a plot of distance (µm) vs time (tdel− ti), see Figure 1. The linear plots suggest as the strain percentage is increased the distance of delamination over a 24 hour period increases. Scanning electron microscope (SEM) images, shown in Figure 2, display cracking of the trivalent chromium coating. The availability of the underlying iron is estimated by studying the surface controlled hydrogen evolution curves. Increasing the uniaxial strain produced higher values for hydrogen evolution. It is proposed that the increase in hydrogen evolution levels can be related to an increase in the exposure of iron caused by the cracking formations found on the surface. The cracking shown on the surface of the trivalent chromium coated steel (Figure 2) is also suggested to cause the increase in cathodic disbondment rates found during the SKP investigations. Cracking of the surface and exposure of iron is suggested to allow for a continuous cathodic front, causing an ingress of electrolyte underneath the PVB model coating. This investigation displays a link between surface cracking, caused by deformation, and the delamination rates of organic coatings on trivalent chromium coated steels. Figure 1

Evaluation of multi-layered graphene nano-platelet composite coatings for corrosion control part II – Cathodic delamination kinetics

In-situ Scanning Kelvin probe (SKP) measurements are used to follow the corrosion-driven cathodic delamination kinetics of model coatings comprising graphene nano-platelets (GNP) dispersed in polyvinylbutyral (PVB) adherent to iron and zinc (galvanised steel). To reduce delamination rates by >90% (relative to unpigmented PVB) a GNP volume fraction of 0.056 is required on iron but only 0.028 on zinc. On this basis, together with work function and O2 permeability data it is proposed that the GNP acts principally to slow through-coating oxygen transport on iron; whereas on zinc a galvanic couple forms between zinc and GNP, displacing cathodic oxygen reduction.