Materials Performance Supplements

CORTEC 2017

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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Particle size influences many properties and is a valuable indicator of performance. This is true for powders, suspensions, emul- sions, and aerosols. The size and shape of powders inf luences f low and compaction properties. Larger, more spherical particles will typically flow more easily than smaller particles that tend to agglomerate. 6 Smaller particles dissolve more quickly and lead to higher suspension viscosities than larger ones. In the case of a VCI, size also matters. Figure 2 shows a comparison of parti- cles, contaminants, and the agglomeration of small particles, all of which can influence the effectiveness of corrosion protection. Nanotechnology deals with structures that are 100 nm or smaller and involves devel- oping materials or devices within that size range. To improve corrosion performance, nano-size powders are incorporated into VCIs. Particle deposition is the sponta- neous attachment of particles to surfaces. The particles in question are normally col- loidal, while the surfaces involved may be planar, curved, or represent particles much larger in size than the depositing ones. Depositing particles may form a monolayer that inhibits further particle deposition or may facilitate further particle deposition depending on the dynamics. As determined in this research , the VCIs are adsorbed to the metal surface by physiosorption . Ir ving Langmuir devel- oped an adsorption isotherm that models gases adsorbed to solid surfaces. 7-10 It is a semi-empirical isotherm with a kinetic basis and was derived based on statistical thermodynamics. It is the most common isotherm equation to use due to its simplic- ity and its ability to fit a variety of adsorp- tion data. It is based on four assumptions: 1. All of the adsorption sites are equiva- lent and each site can only accommo- date one molecule. 2. The surface is energetically homoge- neous and adsorbed molecules do not interact. 3. There are no phase transitions. 4. At the maximum adsorption, only a monolayer is formed. Adsorption only occurs on localized sites on the surface, not with other adsorbates. Some of the criteria assumed in Lang- muir's model do not fit with the obser - vation s from thi s inv e sti gation . Th ere are always imperfections on the surface, a d s orb ed m o l e cul e s are n ot n e c e ssar - ily in er t, and th e m echani sm i s clearly not th e sam e for th e first molecules to adsorb to a surface as for the last mole- cules. Another condition has to do with the thickness of particle deposition, and more likely, molecules will adsorb to the initial monolayer. Despite its shortcom- ings, th e L angmuir i soth erm has many applications in surface kinetics. For the Brunauer Emmett Teller (BET) Model (an isotherm developed by Stephen Brunauer, Paul Emmett, and Edward Teller), given that the four assumptions made to fit the Langmuir isotherm model are, in general, not realistic, it is assumed that the mol- ecules may form multilayers (Figure 3). However, BET theor y ignores inhomoge- neities of the surface and lateral adsor - bate-adsorbate interactions. 11 FIGURE 2 Relative size for particles, aggregates, and corrosive species. FIGURE 3 Atomic models for possible types of surface coverage, monolayer or multiple particle deposition on the metal. 11 FIGURE 4 Left: VCI-A nano-size particle. Right: VCI-A coarse particle. 5 CORTEC SUPPLEMENT TO MP MATERIALS PERFORMANCE JUNE 2017

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