Steel priming

Application of protective paints consists of surface preparation of steel, priming coat and finishing coats. Wherever possible, steel should be blast-cleaned before painting. Primers thoroughly wet the metal to promote adhesion of finishing paints and carry inhibitive pigments. For example, red lead oxide will minimize the spread of rust on metal surfaces. The total thickness of fmishing coats must be at least 0.125 mm for adequate protection and life. Four coats of paint usually are necessary to achieve this. 

Paints used for protecting the bottoms of ships encounter conditions not met by structural steelwork. The corrosion of steel immersed in sea-water with an ample supply of dissolved oxygen proceeds by an electrochemical mechanism whereby excess hydroxyl ions are formed at the cathodic areas. Consequently, paints for use on steel immersed in sea-water (pH 8-0-8-2) must resist alkaline conditions, i.e. media such as linseed oil which are readily saponified must not be used. In addition, the paint films should have a high electrical resistance to impede the flow of corrosion currents between the metal and the water. Paints used on structural steelwork ashore do not meet these requirements. It should be particularly noted that the well-known structural steel priming paint, i.e. red lead in linseed oil, is not suitable for use on ships bottoms. Conventional protective paints are based on phenolic media, pitches and bitumens, but in recent years high performance paints based on the newer types of non-saponifiable resins such as epoxies. 

Good adhesion to the metal. The coating must have an excellent bond to steel. Priming systems are frequently used to assist adhesion. 

The effect of alkoxide primers on crack propagation in the adhesively bonded joints is shown in Figs 1-5. The effect of alkoxide primer on crack propagation for PES-bonded steel primed at 34% RH and tested by immersion in DI water at 100°C for 200 h is illustrated in Fig. 1, plotted as crack length (in cm) vs. time (in h). The initial crack length for both the A1TSB and AITTB primed samples was... 

Figure 1. Crack length as a function of time for mild steel primed at 34% RH and bonded with PES and placed in a 100 C water bath.

SAFETY PROFILE Confirmed carcinogen. Mutation data reported. When heated to decomposition it emits toxic fumes of ZnO and K2O. Used as a corrosion inhibiting pigment and in steel priming. See also CHROMIUM COMPOUNDS and ZINC COMPOUNDS. 

Hazardous Decomp. Prods. Heated to decomp., emits toxic fumes of ZnO and K2O Uses Anticorrosion pigment in primer paints zinc green pigment component in steel priming... 

Calcium oxide Cupric sulfate anhydrous Cupric sulfate pentahydrate Di-iron phosphide steel mfg., specialty Calcium silicon steel priming Zinc potassium chromate steel processing Bismuth... 

Common alloying elements include nickel to improve low temperature mechanical properties chromium, molybdenum, and vanadium to improve elevated-temperature properties and silicon to improve properties at ordinary temperatures. Low alloy steels ate not used where corrosion is a prime factor and are usually considered separately from stainless steels. 

Standard Wrought Steels. Steels containing 11% and more of chromium are classed as stainless steels. The prime characteristics are corrosion and oxidation resistance, which increase as the chromium content is increased. Three groups of wrought stainless steels, series 200, 300, and 400, have composition limits that have been standardized by the American Iron and Steel Institute (AlSl) (see Steel). Figure 8 compares the creep—mpture strengths of the standard austenitic stainless steels that are most commonly used at elevated temperatures (35). Compositions of these steels are Hsted in Table 3. 

A Russian ammonia pipeline of nearly 2400 km extends from Togliatti on the Volga River to the Port of Odessa on the Black Sea, and a 2200-km, 250-mm dia branch line extends from Godovka in the Ukraine to Panioutino. The pipeline is constmcted of electric-resistance welded steel pipe with 7.9-mm thick walls but uses seamless pipe with 12.7-mm thick walls for river crossings. The pipeline is primed and taped with two layers of polyethylene tape and suppHed with a cathodic protection system for the entire pipeline. Mainline operating pressure is 8.15 MPa (1182 psi) and branch-line operating pressure is 9.7 MPa (1406 psi) (11). 

Tsai et al. have also used RAIR to investigate reactions occurring between rubber compounds and plasma polymerized acetylene primers deposited onto steel substrates [12J. Because of the complexities involved in using actual rubber formulations, RAIR was used to examine primed steel substrates after reaction with a model rubber compound consisting of squalene (100 parts per hundred or phr), zinc oxide (10 phr), carbon black (10 phr), sulfur (5 phr), stearic acid (2 phr). 

Polished steel substrates primed with plasma polymerized acetylene films were immersed into a stirred mixture of these materials at a temperature of 155 5°C to simulate the curing of rubber against a primed steel substrate. During the reaction, the mixture was continuously purged with nitrogen to reduce oxidation. At appropriate times between 1 and 100 min, substrates were removed from the mixture, rinsed with hexane ultrasonically for 5 min to remove materials that had not reacted, dried, and examined using RAIR. The RAIR spectra obtained after reaction times of 0, 15, 30, and 45 min are shown in Fig. 13. 

---