Ashok Sabata

The interphase in painted metals pretreated by functional silanes

A new post-rinse treatment was developed to replace the standard chromate post-rinse treatment on phosphated metal substrates. The silicate/silane (SSL) treatment modifies the paint-phosphate interface to give improved adhesion and corrosion performance. Time-of-flight secondary ion mass spectrometry (TOFSIMS) was used to study the orientation of the silane. The orientation was critical for the final adhesion performance. Electrochemical impedance spectroscopy (EIS), an ideal technique for studying interfaces nondestructively, showed that the SSL treatment was effective in reducing the corrosion significantly. Mechanisms of adhesion and corrosion are described.

Characterization of films of organofunctional silanes by TOFSIMS and XPS

The structures of thin films formed by the silanes N-[2-(vinylbenzylamino)-ethyl]-3-aminopropyltrimethoxysilane (SAAPS) and γ-aminopropyltriethoxysilane (γ-APS) deposited onto mechanically polished zinc or mild steel from dilute aqueous solutions were determined using time-of-flight (TOF) SIMS and XPS. TOFSIMS gave structural information which was highly complementary to the XPS data. Aspects such as silane condensation and crosslinking, oxidation at elevated temperatures, the formation of metallosiloxane bonds, and incomplete hydrolysis were detected by TOFSIMS by virtue of its high mass resolution and unlimited mass range. The structures of the films were found to be strongly dependent on the nature of the substrate, the deposition conditions, and heat treatment of the films.

Testing the adhesion of paint films to metals by swelling in N-methyl pyrrolidone

A new test for estimating the adhesion of paints to metal substrates is presented. Small painted disks are immersed in N-methyl pyrrolidone (NMP) at 60°C and the time for the paint film to dclaminate completely and intact from the substrate is recorded. This time, termed the NMP paint retention time or NMPRT, is shown to be sensitive to changes in the metal pretreatment, the type of paint, the cure conditions, and the presence of water at the interface. As the strong swelling of the paint in NMP induces interfacial shear stresses, the NMPRT value is believed to be a measure of the number or strength of the interfacial bonds. Two examples are discussed in some detail: (i) delamination of automotive epoxy-based electrocoat systems from phosphated cold-rolled steel (CRS) and electrogalvanized steel (EGS), and (ii) modification of the interface between stainless steels (types 301 and 409) and epoxy or polyester powder paint systems by means of organofunctional silanes. In both examples it is shown how the NMP...

Paint delamination from electrocoated automotive steels during atmospheric corrosion. Part I. Hot-dip galvanized and electrogalvanized steel

—The mechanism of corrosion-induced paint delamination of electrocoated electrogalvanized and hot-dip galvanized automotive steels has been investigated. The materials were phosphated and electrocoated but not topcoated. The scribed panels were exposed to atmospheric conditions for 15 months. Two distinct regions were observed in cross-sectioned samples: a delamination zone without corrosion products which preceded a zone where the coating forms corrosion products, which consist largely of ZnO/Zn(OH)2. The mechanism of delamination is postulated to be mechanical in nature.

Under-Vehicle Corrosion Testing of Primed Zinc and Zinc Alloy-Coated Steels

Abstract The mechanism of underfilm corrosion propagation and the type of corrosion products formed in electrocoated zinc and zinc alloy coated steels exposed in an under-vehicle corrosion test (UC...

Metal Surface Preparation by Plasma-Polymerized Films

Thin films (< 1 000 A thickness) were deposited by polymerization in an RF plasma of hexamethyldisiloxane (HMDS) and ethylene on cold-rolled steel (CRS), electrogalvanized steel (EGS) and on polished pure zinc substrates. Prior to deposition the metals were cleaned in a reactive or nonreactive plasma in the same reactor. The films were characterized by various analytical techniques (FTIR, TOFSIMS, XPS, and AES) for structure, composition, and metal-polymer interfacial composition. All plasma films appeared to be highly polymerized, crosslinked, and virtually insoluble in organic solvents. The ethylene films also showed a high degree of aromaticity and unsaturation. Both types of films afforded excellent resistance of the metal substrate to corrosion in humidity tests at elevated temperatures. The films were stable in air at temperatures up to at least 175°C. Some films provided corrosion resistance even after severe deformation by deep drawing a 5 cm diameter dome in the panel. The corrosion resistance of the film deposited on zinc substrates was usually better than that of the same films on CRS. Analysis by AES indicated that the film-metal interface compositions for these two metals were different. These differences will be related to the surfaces of the substrates

TOFSIMS Studies of Cleaning Procedures and Silane Surface Treatments of Steels

Surface contaminants can affect properties such as wet-tability and reactivity in subsequent surface preparation processes. Cleaning of metal substrates has usually been studied by techniques such as AES and XPS. These techniques do not give any information on the molecular structure of the contaminants; however, Time-of-Flight Secondary Ion Mass Spectrometry (TOFSIMS) with its monolayer sensitivity, has been effective in providing structural, molecular information of organic contaminants which may be useful in identifying the source of the contamination. Industrial alkaline cleaning and degreasing processes leave mainly a siloxane-type of contaminant that deteriorates the adhesion of metals to epoxies. Phosphating leaves the surface clean and microrough, which improves the adhesion of adhesives to cold-rolled steel (CRS). Epoxies when cured at high temperatures give better adhesion values as compared to room temperature epoxies. Organofunctional silanes such as styrylamino aminopropyltrimeth-oxysilane (SAAPS) were found to have a specific orientation on the metal substrate as a function of cleanliness and mode of application. The orientation of the vinylbenzyl group towards the epoxy adhesive is shown to improve adhesion. A combined silicate and silane post-rinse of phosphate coatings is shown to be effective in giving the silanes the required orientation for bonding to epoxies, thus improving the adhesion.

Modification of the Interface between Paints and Stainless Steels by Means of an Interphase of Crosslinked Organofunctional Silanes

The adhesion of paint systems to stainless steel substrates can be markedly improved by a pretreatment using organofunctional silanes. The paint-metal interface thus obtained can be made more resistant to corrosive attack by adding a silane crosslinker to the hydrolyzed silane solution. The films formed on the metal surface were characterized by Time-of-Flight Secondary Ion Mass Spectrometry (TOFSIMS).

Surface segregation in blends containing poly(dimethylsiloxane)-polystyrene block copolymers

The surface composition of polystyrene blends containing poly(dimethylsiloxane)-polystyrene block copolymers have been analyzed using X-ray photoelectron spectroscopy (XPS), contact angle measurements, and time-of-flight secondary ion mass spectrometry (TOFSIMS). The three techniques showed the surface of the blend samples to be identical to pure poly(dimethylsiloxane) homopolymer, despite the fact that the systems each contained only a 2% bulk concentration of siloxane. The high surface sensitivity of TOFSIMS—which probes the samples to depths of a few angstroms—indicates an enrichment of-Si(CH3)3 groups at the surface. These are the terminal groups of the PDMS part of the block. Their enrichment at the surface of the samples is presumably due to their low surface energy, in addition to the tendency for end groups to be at the surface due to free volume considerations.

Chemical stability of phosphate conversion coatings on cold-rolled and electrogalvanized steels

Summary The attack of phosphate conversion coatings on cold-rolled (CRS) and electrogalvanized (EG) automotive steels in high and low pH solutions has been studied. Hopeite (HP) and phosphophyllite (PPH) crystals dissolve completely at pH below 3. HP also dissolves in alkali, whereas PPH is converted to a mixed oxide. The phosphate crystals have a pronounced texture which is influenced by the texture of the substrate. Certain orientations are more stable at high pH than others. There is a strong effect of the temperature on the chemical stability of the phosphate at high pH. The presence of NaCl increases the rate of phosphate attack at high pH. The mechanism of attack at high pH is a solid state reaction forming Zn(OH) 2 which is stabilized by iron, manganese and other ions resulting in the formation of complex zinc hydroxy compounds.