Henry Leidheiser

The mechanism for the cathodic delamination of organic coatings from a metal surface

Abstract A detailed mechanism of cathodic delamination from a defect in an organic coating is presented, which takes into account the reactants and how they get to the delaminating front. Oxygen and water pass through the organic coating whereas cations, at least in part, reach the front by lateral diffusion. Five means for minimizing cathodic delamination are suggested: (1) No bare metal, or superficially oxidized metal, should be present at the coating/substrate interface. (2) There should exist at the coating/substrate interface a layer which is a very poor conductor of electrons. (3) The interfacial layer at the coating/substrate interface should be a poor catalyst for the cathodic reaction. (4) The boundary between the substrate and the coating should be rough in order to provide a tortuous path for lateral diffusion. (5) The interfacial region should be resistant to alkaline attack.

Electrical and electrochemical measurements as predictors of corrosion at the metal—organic coating interface

Abstract Five types of electrical and electrochemical measurements are reviewed: D.C. electrical properties, A.C. resistance, capacitance, corrosion potential, and polarization curves. When the coating resistance decreases with time and falls below 10 6 to 10 7 ohms/cm 2 , corrosion beneath the coating is occurring at a significant rate. A low electrical resistance requires (a) ion and water penetration into the coating, (b) ready motion of ions through the coating, and (c) proceeding ionic/electron transfer reactions (corrosion) at the polymer—metal interface. Care must be taken when making electrical measurements that the measurement itself does not affect the corrosion rate.

Mössbauer spectroscopic study of the formation of Fe(III) oxyhydroxides and oxides by hydrolysis of aqueous Fe(III) salt solutions

Abstract Mossbauer spectroscopy has been used to investigate the precipitates formed by hydrolysis of 0.1 M solutions of Fe(NO 3 ) 3 , FeCl 3 , Fe 2 SO 4 ) 3 , and NH 4 Fe(SO) 2 at 90°C. The isomer shifts, electric quadrupole splittings, and nuclear magnetic splittings were used for the qualitative and quantitative identification of the hydrolysis products. Proposals were made concerning the mechanism of formation of the oxides and hydroxyoxides of iron. Hydrolysis in the nitrate and chloride solutions proceeds by the formation of monomers and dimers of iron III) ions, followed by the formation of polymeric species. The polymers formed in the nitrate solution are not presumed to include the nitrate ion in the polymer chain, whereas the polymers formed in the chloride solution contain some chloride ions in place of the hydroxyl ion. The next step in the precipitation process is the formation of oxybridges and the development of α-FeOOH and β-FeOOH structures. This step is followed by loss of water and internal crystallization of α-FeOOH to α-Fe 2 O 3 in nitrate solution or by dissolution of β-FeOOH and growth of α-FeOOH in chloride solution. In sulfate solutions the formation of an FeSO 4 + complex suppresses the polymerization process and the formation of oxyhydroxides and oxides. Basic Fe(III) sulfates are formed instead.

Cathodic delamination of polybutadiene from steel-A review

The variables influencing the rate of the cathodic delamination of polybutadiene from steel in alkali halide solutions are discussed. The bond at the interface arises from the reaction of carboxyl groups in the polybutadiene with hydroxyl groups on the surface of the iron oxide at the interface. It is this bond that is broken by hydroxyl ions generated by the oxygen reduction reaction beneath the coating. The rate controlling step is the rate of charge transfer through the coating to support the oxygen reduction reaction.

The atmospheric corrosion of iron as studied by Mössbauer spectroscopy

Abstract The composition of the rust on the surface of steel panels was determined after atmospheric exposure times of 2 weeks, 2 months and 6 months. The initial product is γ-FeOOH which converts in time to a mixture of α-FeOOH and γ-Fe2O3. Steel exposed for times of the order of 25 years is covered with corrosion product consisting largely of γ-Fe2O3. The similarity between the composition of the corrosion products and precipitates formed from FeSO4 solution under mildly acidic conditions at 90°C suggests that the dominant anion in these atmospheric corrosion experiments is sulfate.

A Mössbauer spectroscopic study of rust formed during simulated atmospheric corrosion

Abstract Steel coupons were subjected to 100% relative humidity and were inoculated every day with 100 μl of 0.01 N solutions of NaCl, Na 2 SO 4 , LiCl or CsCl. The first solid rust constituent that formed contained significant amounts of both γ-FeOOH and ferrihydrite. In contrast, only γ-FeOOH was observed in the rust formed during atmospheric corrosion and during wet-dry cycling with distilled water in the laboratory. The ferrihydrite in time was converted to γ-FeOOH, α-FeOOH, and γ-Fe 2 O 3 . The fractions of ferrihydrite + γ -FeOOH in the rust formed as a function of time during atmospheric exposure and during rusting in the laboratory environment were the same in the two cases.

Chemistry of the Metal-Polymer Interfacial Region

In many polymer-metal systems, chemical bonds are formed that involve metal-oxygen-carbon complexes. Infrared and M�ssbauer spectroscopic studies indicate that carboxylate groups play an important role in some systems. The oxygen sources may be the polymer, the oxygen present in the oxide on the metal surface, or atmospheric oxygen. Diffusion of metal ions from the substrate into the polymer interphase may occur in some systems that are cured at elevated temperatures. It is unclear whether a similar, less extensive diffusion occurs over long time periods in systems maintained at room temperature. The interfacial region is dynamic, and chemical changes occur with aging at room temperature. Positron annihilation spectroscopy may have application to characterizing the voids at the metal-polymer interface.

Emission Mössbauer Studies of Anodically Formed CoO2

was formed by anodic treatment of deposited on a platinum electrode in a borate buffer at +600–+900 mV (vs. SCE). Anodic polarization of a cobalt electrode at +900 mV yielded a mixture of and . Exposure of to air for 3 months resulted in decomposition to .

Surface properties of Ni(OH)2 and NiO. I. Water adsorption and heat of immersion of Ni(OH)2

Nickelous hydroxide samples were prepared from a nickelous nitrate solution with distilled ammonia gas. Particle sizes by X-ray diffraction and electron microscopy ranged from 40 to 400 nm. Water adsorption isotherms, measured gravimetrically, indicated that hydrophilicity depends upon sample preparation. The isosteric heats of adsorption calculated for one sample from multi-temperature isotherms at 25 and 39°C showed high heats of adsorption, 69 kJ/mole, at coverages below 4 molecule/nm2. The heat of immersion of the same sample of Ni(OH)2, measured by means of an adiabatic calorimeter as a function of water precoverage, showed that the heat decreased linearly from an initial zero water coverage value of 320 to 190 mJ/m2 at a coverage of 2.9 molecule/nm2. It is concluded that the surface of Ni(OH)2 is predominately hydrophobic with a limited concentration of polar sites on the crystal edges available for hydrogen bonding with polar adsorbates.

Surface properties of Ni(OH)2 and NiO. II. Mechanism for the thermal decomposition of Ni(OH)2 and other metal hydroxides

Abstract Nickelous hydroxide has the CdI2 structure and can be converted to NiO with the NaCl structure by thermal decomposition. It is proposed that the hexagonal plates of the hydroxide split into layers of the oxide when thermally decomposed at moderate temperatures. The relationship between the specific surface areas of the product NiO and its parent Ni(OH)2 is Σ 2 Σ 1 = 2 H 2ρ2 (1− h rn ) 1 Σ 1 + ρ 1 rρ 2 whereΣ is the specific surface area, H2 is the NiO plate thickness, ρ is the density, n is the number of layers which the parent plate will yield, h and r are crystallographic-geometric factors, and subscripts 1 and 2 refer to the hydroxide and oxide, respectively. Five Ni(OH)2 samples, which were prepared through precipitation by NH3 gas or by direct addition of NaOH, were decomposed in vacuum at 200°C. The specific surface areas of the oxide and its parent fit the above equation excellently. A review of the literature yielded surface area data for other samples of hydroxide precipitated from the ammonia-nickel complex which also fit the equation but with a higher slope and intercept. Electron micrographs provide support for the proposed splitting of the hydroxide plate. The magnesium hydroxide/magnesium oxide system is similar to the nickel system. A review of the literature yielded surface area data which also follow the above relationship. The slope provides information on the thickness of the oxide plates. Results are in excellent agreement with published X-ray broadening studies of MgO.