Marine atmospheric corrosion

Fig. 12.9 Corrosion resistance of tin-nickel electrodeposit impaired by pseudomorphic porosity originating on cold-rolled steel surface (left). Panel on right has had the shattered grain surface removed by chemical polishing (0-125 iim removed). Coating thickness 15 iim-, panels exposed 6 months to marine atmospheric corrosion (Hayling Island)...

In tropical marine atmospheres, corrosion is much greater in sea spray conditions, primarily because of the higher temperature. It would seem that magnesium salts present in the tropics (at a level similar to that in temperate waters) are insufficient to inhibit corrosion. The dependence of zinc corrosion on the distance from the coast was discussed earlier. 

Kain, R. M., and Baker, E. A. (1989). Marine atmospheric corrosion museum report on the performance of thermal sprayed coatings on steel. ASTM STP 947, ASTM, Philadelphia, pp. 211-234. 

MorciUo M, Chico B, Mariaca L, Otero E. Salinity in marine atmospheric corrosion Its dependence on the wind regime existing in the site. In Corrosion Science 2000 91-104. 

Experience over more that 50 years acquired both in outdoor corrosion testing stations and by monitoring applications shows that the resistance to marine atmospheric corrosion of unprotected alloys of the 1000, 3000, 5000 and 6000 series is similar and this corrosion develops in the same manner over a period of 20 years or more (Figure C.5.6). 

G. Haynes, R. Baboian, Electrochemical Observations as Related to Marine Atmospheric Corrosion of Chrome-Flashed Stainless Steel, J. Electrochemical Soc. 132... 

Atmospheric corrosion results from a metal s ambient-temperature reaction, with the earth s atmosphere as the corrosive environment. Atmospheric corrosion is electrochemical in nature, but differs from corrosion in aqueous solutions in that the electrochemical reactions occur under very thin layers of electrolyte on the metal surface. This influences the amount of oxygen present on the metal surface, since diffusion of oxygen from the atmosphere/electrolyte solution interface to the solution/metal interface is rapid. Atmospheric corrosion rates of metals are strongly influenced by moisture, temperature and presence of contaminants (e.g., NaCl, SO2,. ..). Hence, significantly different resistances to atmospheric corrosion are observed depending on the geographical location, whether mral, urban or marine. 

The relative susceptibHity of several commercial aHoys is presented in Table 8. The index used is a relative rating based on integrating performance in various environments. These environments include the harsh condition of exposure to moist ammonia, Hght-to-moderate industrial atmospheres, marine atmosphere, and an accelerated test in Mattsson s solution. The latter testing is described in ASTM G30 and G37 (35,36) and is intended to simulate industrial atmospheres. The index is linear. A rating of 1000 relates to the most susceptible and zero designates immunity to stress corrosion. 

Tin—Nickel. AHoy deposits having 65% fin have been commercially plated siace about 1951 (135). The 65% fin alloy exhibits good resistance to chemical attack, staining, and atmospheric corrosion, especially when plated copper or bron2e undercoats are used. This alloy has a low coefficient of friction. Deposits are solderable, hard (650—710 HV ), act as etch resists, and find use ia pfinted circuit boards, watch parts, and as a substitute for chromium ia some apphcafions. The rose-pink color of 65% fin is attractive. In marine exposure, tin—nickel is about equal to nickel—chromium deposits, but has been found to be superior ia some iadustfial exposure sites. Chromium topcoats iacrease the protection further. Tia-nickel deposits are bfitde and difficult to strip from steel. Temperature of deposits should be kept below 300°C. 

There are many special factors controlling atmospheric bimetallic corrosion that entitle it to separate treatment. The electrolyte in atmospheric corrosion consists of a thin condensed film of moisture containing any soluble contaminants in the atmosphere such as acid fumes from industrial atmospheres and chlorides from marine atmospheres. This type of electrolyte has two characteristics which are summarised in a paper by Rosenfel d . 

Sulphur oxides These (SO2 is the most frequently encountered oxide) are powerful stimulators of atmospheric corrosion, and for steel and particularly zinc the correlation between the level of SO2 pollution and corrosion rates is good However, in severe marine environments, notably in the case of zinc, the chloride contamination may have a higher correlation coefficient than SO2. 

In atmospheric exposure 18% Ni maraging steel corrodes in a uniform manner , and becomes completely rust covered. Pit depths tend to be more shallow than for the low-alloy high-strength steels. Atmospheric corrosion rates in industrial (Bayonne, New Jersey) and marine (Kute Beach, North Carolina) atmospheres are compared with those for low-alloy steel in Figs. 3.29, 3.30 and 3.31. The corrosion rates drop substantially after the first year or two and in all cases the rates for maraging steel are about half the corrosion rate for HY80 and AISI 4340 steels. 

In marine atmospheres the overall rates of corrosion are reduced progressively with increase in nickel content up to about 35%, but with small improvement thereafter. The rates of corrosion at various sites, reported by... 

Table 3.31 Resistance of Fe-36 Ni and mild steel to corrosion in marine atmospheres ...

The average rates of corrosion of Fe-36Ni alloy exposed to alternate immersion in sea-water are appreciably greater than those that occur when the alloy is exposed to marine atmospheres. Although the rates of corrosion are significantly below those observed for mild steel (Table 3.32) the superiority over mild steel in not so great with respect to pitting attack. 

In the tests described by Tracy, a high-tensile brass suffered severe dezinc-ification (Table 4.11). The loss in tensile strength for this material was 100% and for a non-arsenical 70/30 brass 54% no other material lost more than 23% during 20 years exposure. In Mattsson and Holm s tests the highest corrosion rates were shown by some of the brasses. Dezincification caused losses of tensile strength of up to 32% for a P brass and up to 12% for some of the a-P brasses no other materials lost more than 5% in 7 years. Dezinc-ification, but to a lesser degree, occurred also in the a brasses tested, even in a material with as high a copper content as 92%. Incorporation of arsenic in the a brasses consistently prevented dezincification only in marine atmospheres. 

In marine atmospheres magnesium chloride is formed and eventually oxychloride by reaction with magnesium hydroxide formed at the same time. Since the chloride is hygroscopic, moisture is attracted and the corrosive effect is hence much worse than that of water alone. 

The purity of the zinc is unimportant, within wide limits, in determining its life, which is roughly proportional to thickness under any given set of exposure conditions. In the more heavily polluted industrial areas the best results are obtained if zinc is protected by painting, and nowadays there are many suitable primers and painting schemes which can be used to give an extremely useful and long service life under atmospheric corrosion conditions. Primers in common use are calcium plumbate, metallic lead, zinc phosphate and etch primers based on polyvinyl butyral. The latter have proved particularly useful in marine environments, especially under zinc chromate primers . 

Sulphates and chlorides are present in industrial and marine atmospheres. In water they accelerate the corrosion of steel. Avoiding lodgement areas for water and dirt reduces the risk of the latter acting as a poultice in which the corrosive salts can build up. 

Conditions within a few hundred metres of the surf line on beaches are intermediate between total immersion in sea-water and normal exposure to a marine atmosphere. High corrosion rates can occur on some tropical surf beaches where the metal remains wet and where inhibiting magnesium salts are not present in the sea-water. 

The hardness of such coatings may reach a maximum of about 400 Hy as compared with approximately 50 Hy for a soft gold deposit. A series of corrosion studies in industrial and marine atmospheres by Baker" has indicated that the protective value of hard gold coatings is comparable with that of the pure metal, and that a thickness of only 0-002 5 mm gives good protection to copper base alloys during exposure for six months. 

S mm and which, as indicated earlier, places strict limitations on the usefulness of the coating for protection against severely corrosive liquid environments. The value of rhodium in resisting atmospheric corrosion in environments ranging from domestic to marine and tropical exposure has, however, been amply demonstrated by experience, and it appears probable that further developments in technology may lead to still wider application. 

Paint for structural steelwork is required mainly to prevent corrosion in the presence of moisture. In an industrial atmosphere this moisture may carry acids and in a marine atmosphere this moisture may carry chlorides. Paint is therefore required to prevent contact between steel and corrosive electrolytes, and to stifle corrosion, should it arise as a result of mechanical damage or breakdown of the coating through age and exposure. 

An important industrial interest is in the corrosion of metals and ceramics by molten sodium sulphate/vanadate solutions. This is because turbines, which are usually nickel-based alloys, operating in a marine atmosphere, containing... 

D. C. Cook, A C. Van Orden, J. Reyes, S.J.Oh, R. Balasubramanian, J.J. Carpio, H.E. Townsend, Atmospheric Corrosion in Marine Environments along the Gulf of Mexico, Marine Corrosion in Tropical Environments, ASTM STP 1399, S.W. Dean,... 

It is common lo consider that certain salts have a very corrosive action. This is true in the respect that the corrodibility of marine atmospheres has been shown to be greater than rural, tropical, and urban atmospheres. For example, ammonium sulfate and ammonium chloride being salts of strong acids and a weak base, that is ammonium hydroxide, hydrolyze in Water to yield the respective acids. These sails then have a corrosive action, which is due actually to the acid produced in hydrolysis. 

Hot corrosion32 is encountered in the operation of gas turbines between 730 and 1730°C. Operation of gas turbine engines in marine atmosphere is prone to hot corrosion, which involves oxidation and reaction with sulfur, sodium, vanadium and other contaminants present in the fuel or ilnet air. The consequence of this is the loss of protective action of chromium oxide on the blade and sometimes engine failure. 

CLIMAT (classification of Industrial and Marine Atmospheres,21 gives data on uniform or galvanic corrosion of aluminum wire wound around threaded bolts made of steel or copper in an atmosphere of interest)... 

Figure 4.4 Schematic illustration of the corrosion product layers identified on steels exposed to rural and marine atmospheres for periods of up to 5 yr...

The source of chloride is seawater as well as the deicing salts used on roads. The hydrogen chloride is also present in seawater aerosols. The corrosion of copper and its alloys in marine atmospheres has been studied and a corrosion rate of 600-700 pg/cm2 yr averaged over a period of 8 yr has been reported.50... 

Atmospheric corrosion of lead involves exposure to industrial, rural and marine environments. The mode of corrosion in the three environments is different. The rural environment consists of humidity, airflow and rainfall, which may be considered to be innocuous. The marine environment consists of chloride entrained in air and could... 

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