**2. Materials and Methods**

## *2.1. Materials and Epoxy Coatings*

The steel test material was cold rolled steel (DC 01—ASTM A366 [20] with a chemical composition max %, 0.12 C, 0.045 P, 0.045 S, 0.60 Mn, and the specimens were cut from a plate of 0.3 cm thickness. The dimensions of the test coupons were 5 cm × 1.5 cm, the total exposed area for the salt spray tests was 15 cm2, and for the electrochemical measurements was 6 cm2. The steel surface before coating was mechanically cleaned by scrubbing with a bristle brush and chemically cleaned using acetone and alcohol.

The metallic specimens were coated with a ~20 μm thin film of pristine glassy epoxy polymer or epoxy-clay nanocomposite films. No pinholes or other defects were observed on the coatings. The pristine liquid epoxy resin was diglycidyl ether of bisphenol A (DGEBA) (EPON 828RS, Hexion, Columbus, OH, USA) with an average epoxide equivalent weight of ~187 (*M*<sup>W</sup> = 370) and was mixed at 50 ◦C with the appropriate amount of an aliphatic polyoxypropylene diamine (Jeffamine D-230, *M*<sup>W</sup> ≈ 230, Huntsman, The Woodlands, TX, USA), which acted as the curing agent. The molecular structures of the epoxy resin and diamine curing agent are shown in Figure 1A,B, respectively.

**Figure 1.** Molecular structure of (**A**) diglycidyl ether of bisphenol-A (DGEBA) and (**B**) of aliphatic polyoxypropylene diamine (Jeffamine®).

The curing agent was mixed with the epoxy resin under a stoichiometric ratio of 1:1 of amine reactive hydrogens to epoxy rings. The metallic specimens were dipped in the liquid uncured mixture and then were kept in a vertical position so that the excess liquid was removed and left to cure at ambient conditions for 24 h. Final post-curing was performed at 75 ◦C for 3 h and 125 ◦C for another 3 h. The same procedure was applied for the coating of the specimens with the epoxy-clay nanocomposites, except that prior to adding the curing agent, the epoxy pre-polymer (DGEBA) was mixed with the (organo) clay for 1 h at 50 ◦C. The clay loading in the nanocomposites was 3 and 6 wt % on a silicate basis.

The clays used for preparing the nanocomposite coating were the Nanomer I.28E and the Nanomer I.30E, both kindly provided by Nanocor Inc. (Hoffman Estates, IL, USA), which are montmorillonite clays that have been modified with quaternary and primary octadecylammonium ions, respectively. The parent, inorganic Na+-PGW clay (Polymer Grade Wyoming, Nanocor Inc.) was also used for preparing specimens of epoxy-clay nanocomposites for comparing the structure and properties of the bulk nanocomposites samples. All three clays used were polymer grade (PG) montmorillonites which are high purity aluminosilicate minerals with the theoretical chemical formula: M<sup>+</sup> *<sup>y</sup>*(Al2−*<sup>y</sup>* Mg*y*)(Si4)O10(OH)2·*n*H2O. The chemical composition, as measured by Inductively Coupled Plasma Atomic Emission Spectroscopy, ICP-AES, chemical analysis, of the Na+-PGW montmorillonite clay and the two organoclays is presented in Table 1 below. Obviously, there is a dramatic reduction of Na<sup>+</sup> cations in the composition of the organoclays.

The average particle size of Na-PGW as measured by a laser particle size analyzer (Mastersizer S, Malvern Instruments, Malvern, UK) in 0.4 wt % aqueous suspensions was ~2 μm (with a distribution of 0.5–10 μm). Various relevant physicochemical properties of the clays used, as provided by Nanocor Inc., are given in Table 2.


**Table 1.** Data of ICP-AES chemical analysis of parent sodium montmorillonite PGW, I.28E quartenary octadecyl ammonium organoclay, and I.30E primary octadecyl ammonium organoclay.



Notes: Nanocor Inc. Vol. Lit. G-105 "POLYMER GRADE MONTMORILLONITES" (Nanocor, 2006); Nanocor Inc. Vol. Lit. T-11 "Epoxy Nanocomposites Using Nanomer® I.30E Nanoclay" (Nanocor, 2004); Nanocor Inc. Vol. Lit. T-12 "Nanocomposites Using Nanomer® I.28E Nanoclay" (Nanocor, 2004).
