Nanoparticle-filled silane films as chromate replacements for aluminum alloys

Abstract

Silane surface treatments have been developed as an alternative for toxic and carcinogenic chromate-based treatments for years. It is consistently observed that ultra-thin silane films offer excellent corrosion protection as well as paint adhesion to metals. The silane performance is comparable to, or in some cases better than, that of chromate layers. The most recent studies also showed that the silane films can be thickened and strengthened by loading of a small amount of nanoparticles such as silica and alumina into the films resulting in enhanced corrosion protection of aluminum alloys.

Introduction

Recently, silane surface treatment has evolved as a promising alternative for toxic chromate-based treatments in metal-finishing industries [1], [2], [3], [4], [5], [6], [7], [8]. The studies concerning corrosion protection of metals using silane films have invariably demonstrated that silanes, a group of environmentally compliant chemicals, could efficiently protect metals against different forms of corrosion in two major ways. That is, (1) a single silane film can protect metals against corrosion without topcoats for 6 months to 1 year, and (2) silanes can also be used in surface pretreatments of metals before painting. In the former case, the thickness of silane films is normally around 200–400 nm obtained from 5% silane solutions. In the latter case, the as-deposited ultra-thin silane layers (∼100 nm) from 2% silane solutions perform excellently as a paint adhesion promoter as well as a corrosion retardant under different topcoats such as epoxies, polyurethanes, polyesters and acrylics.

Trialkoxysilanes (or silanes) with the general formula of R′(CH₂)_nSi(OR)₃ (where R′ denotes organic functionality, and OR indicates hydrolyzable alkoxy group, e.g. methoxy (OCH₃) or ethoxy (OC₂H₅)), have been widely used as adhesion promoters in paints, fillers, and binders in the industry of glass/polymer composites for a long time. The studies of silanes as adhesion promoters have been conducted extensively by the adhesion scientific community in the past few decades. An abundance of silane theories have been well-documented [9], [10], [11]. According to these studies, a general accepted bonding mechanism of silanes to metal surfaces is illustrated in Fig. 1 [12]. When dipping a metal into a dilute silane solution (e.g. 2–5 vol.%) for a few seconds, silanols (SiOH) in the silane solution adsorb spontaneously onto the metal surface through hydrogen bonds, as shown in Fig. 1(a). Upon drying, there are two key condensation reactions occurring at the silane/metal interfacial region. SiOH groups from the silane solution and the metal hydroxyls (MeOH) from the metal surface hydroxide form covalent metallo-siloxane bonds (MeOSi) according to

$$\text{SiOH}{}_{\mathrm{<mtext>(solution)</mtext>}}\text{+}\text{MeOH}{}_{\mathrm{<mtext>(metal</mtext><mspace\; sp="0.25"\; width="2px"\; linebreak="nobreak"\; is="true"></mspace><mtext>surface)</mtext>}}\text{=}\text{SiOMe}{}_{\mathrm{<mtext>(interface)</mtext>}}\text{+}\text{H}{}_{\mathrm{<mtext>2</mtext>}}\text{O}$$

The excess SiOH groups adsorbed on the metals would also condense among themselves to form a siloxane (SiOSi) film

$$\text{SiOH}{}_{\mathrm{<mtext>(solution)</mtext>}}\text{+}\text{SiOH}{}_{\mathrm{<mtext>(solution)</mtext>}}\text{=}\text{SiOSi}{}_{\mathrm{<mtext>(silane</mtext><mspace\; sp="0.25"\; width="2px"\; linebreak="nobreak"\; is="true"></mspace><mtext>film)</mtext>}}\text{+}\text{H}{}_{\mathrm{<mtext>2</mtext>}}\text{O}$$

The as-formed MeOSi and SiOSi covalent bonds are assumed to be responsible for the excellent bonding of the silane film to the metal substrate (Fig. 1).

It has also been demonstrated in the previous studies that bis-silanes such as bis-[3-(triethoxysilylpropyl)]ethane (BTSE, (OC₂H₅)₃Si(CH₂)₂Si(OC₂H₅)₃) and bis-[3-(triethoxysilyl)propyl]tetrasulfide (bis-sulfur silane, (OC₂H₅)₃Si(CH₂)₃S₄(CH₂)₃–Si(OC₂H₅)₃) performed much better in terms of corrosion protection as compared with mono-silanes [2], [3], [4], [5]. The major difference between these two types of silanes is that the number of hydrolyzable OR groups in a bis-silane molecule doubles that in a mono-silane molecule, as illustrated in Fig. 2 [12]. A mono-silane molecule only has three OR groups attached to the silicon (Si) atom at one end (Fig. 2(a)), whereas a bis-silane molecule has six OR groups in total and two Si atoms at both ends, with every three OR groups attached to a Si atom (Fig. 2(b)). It was further noted in our previous studies [2], [3], [4], [5] that the bis-silanes tend to bond to metal substrates more tightly than the mono-silanes. The former with more OR groups is able to develop a much denser interfacial region through the above two reactions (1) and (2) than the latter a more detailed explanation regarding this aspect was given elsewhere [12].

BTSE and bis-sulfur silanes exhibit outstanding corrosion protection performance on Al alloys such as AA 2024-T3, AA 6061-T6, AA 5005 and AA 7075-T6 without topcoats. An example is given in Fig. 3, which shows that bis-sulfur silane-treated AA 2024-T3 panel survived 504 h of salt spray test (SST) without showing any significant corrosion activity on the metal surface (Fig. 3(c)). The untreated and chromated (Alodine^® 2000 process) panels (Fig. 3(a) and (b)) were used here as the controls. Similar to the silane-treated surface, the chromated surface has also been protected very well. Nevertheless, it should be pointed out that the bis-sulfur silane film obtained from a 5% silane solution is about 400 nm thick. While the chromate obtained in the Alodine process has a typical thickness of greater than 1000 nm. Thus, silane films outperform chromate films on a per-weight basis.

In addition to the anticorrosive efficiency, another major concern of the use of silane films is the mechanical properties of such films. In service, these silane films on metals should be capable of resisting mechanical damages by impact, scratch and wear. Therefore, one of our current research subjects is to improve mechanical properties of silane films by loading nanoparticles into the films.

A preliminary study on this aspect was done on AA 5005 treated with a water-based silane (i.e. a mixture of bis-amino silane and vinyltriactoxysilane (VTAS)) loaded with a small amount of alumina nanoparticles obtained from the 5% silane solution containing 50 ppm alumina. The 336 h-SST results are shown in Fig. 4. It is seen that the corrosion protection offered by the nano-structured silane film (Fig. 4(c)) is comparable to that of the chromate layer (Fig. 4(b)). No corrosion has been found on both treated alloy surfaces (Fig. 4(b) and (c)). The untreated surface in Fig. 4(a), on the other hand, has corroded heavily. Most recently, a systematic study has been done by using nanoparticle-loaded bis-sulfur silane films on AA 2024-T3 substrates, where the nanoparticles were colloidal silica. The results are reported here.