CORROSION-PRODUCT FILMS

9.3 CORROSION-PRODUCT FILMS

Corrosion - product fi lms formed in the atmosphere tend to be protective; that is, the corrosion rate decreases with time (Fig. 9.1 ) [3] . This is true to a lesser extent of pure iron, for which the rate is relatively high, compared to the copper - bearing or low - alloy steels, which are more resistant. Rust fi lms on the latter steels tend

T A B L E 9.1. Atmospheric Corrosive Gases in Outdoor Urban Environments [1]

Gas

Approximate Concentration Range (ppbv)

SO 2 3 – 1000 NH 3 1 – 90

HCl

NO 2 0.5 – 300 3 O 0.9 – 600

RCOOH

CORROSION-PRODUC T FILMS

Figure 9.1. Atmospheric corrosion of steels as a function of time in an industrial environment ( C. Larrabee, in Corrosion Handbook , H. H. Uhlig, editor, Wiley, New York, 1948, p. 124 )

to be compact and adherent, whereas on pure iron they are a powdery loose product. The corrosion rate eventually reaches steady state and usually changes very little on further exposure. This is characteristic of other metals as well, as can be seen from data obtained by the American Society for Testing and Materi- als for various metals exposed 10 or 20 years to several atmospheres (Table 9.2 )

[4] . Within the experimental error of such determinations, the rate for a 20 - year period is about the same as that for a 10 - year period. The data in Table 9.2 also show the benefi cial effect of alloying elements, primarily 1.1% Cr and 0.4% Cu, in the low - alloy steel compared to the 0.2% C steel.

Data of Fig. 9.1 follow the relation p = kt n (9.1) where p can be expressed as specimen weight loss (g/m 2 ), or as specimen penetra-

tion ( μ m), during time t (years), and k and n are constants that depend on the metal and on the atmospheric conditions (i.e., climatic and pollution factors) at the test site. In addition to carbon steels, this relation is also found to apply to atmospheric test data for galvanized, aluminized, and 55% Al – Zn coatings on steel [5 – 7] . Values of n typically range from about 0.5 to 1. The constant, n = 1, applies for a linear rate law; that is, the corrosion product fi lm is not protective. In comparison with carbon steels, weathering steels have very low values of n ,

194 ATMOSPHERIC CORROSION

T A B L E 9.2. Average Atmospheric Corrosion Rates of Various Metals for 10 - and 20 - Year

Exposure Times, mils/year a (American Society for Testing and Materials) [4]

Metal

Atmosphere

New York City

La Jolla, CA

State College,

(Urban

(Marine)

PA (Rural)

(Monel) Zinc (99.9%)

0.2% C steel b (0.02% P, 0.05% S,

0.05% Cu, 0.02% Ni, 0.02% Cr) Low - alloy steel b (0.1% C, 0.2% P,

0.04% S, 0.03% Ni, 1.1% Cr, 0.4% Cu)

a 1 mil/year = 0.001 in./year = 0.0254 mm/year = 25.4 b Kearney, New Jersey (near New York City); values cited are one - half reduction of specimen thickness μ m/year. [C. P. Larrabee, Corrosion 9 , 259 (1953)].

usually less than 1 2 [8] . A value of n = 0.5 corresponds to a parabolic

rate law. From (9.1), the linear bilogarithmic law may be expressed as

log p =+ AB log t (9.2)

where A = log k and B = n . The atmospheric behavior of a specifi c metal at a specifi c location can be described using the two parameters A and B . This biloga-

rithmic law can be very useful in predicting long - term atmospheric corrosion damage based on tests of shorter duration. Extrapolation up to 20 – 30 years from tests of 4 years is possible with reasonable confi dence [9] . Over the longer term, changes in the environment are likely to be more signifi cant than deviations from the model.

The mean corrosion rate, p / t , can be calculated as

() t

A B t (9.3) log =+ ( − 1 ) log

FAC TORS INFLUENCING CORROSIVIT Y OF THE ATMOSPHERE

and the instantaneous corrosion rate, dp / dt , can be expressed as

() dt

When there is a linear relationship between log p and log t , there are also linear relationships between log p / t and log t , and between log dp / dt and log t . Metal surfaces located where they become wet or retain moisture, but where rain cannot wash the surface, may corrode more rapidly than specimens fully exposed. The reason for this is that sulfuric acid, for example, absorbed by rust will continue to accelerate corrosion, perhaps by means of the cycle

1 1 1 1 2 4 H SO +

2 O 2 4 O 2 + 2 2 4 H SO

1 2 HO 2 1 3 Fe ⎯ ⎯⎯⎯⎯ → FeSO 4 ⎯ ⎯⎯⎯⎯⎯ → Fe SO 2 ( 43 ) ⎯ ⎯ ⎯⎯ → Fe O 2 3 +

2 H SO 4

Intermediate formation of ferric sulfate has not been demonstrated, and so FeSO 4 may oxidize directly to Fe 2 O 3 . Nevertheless, rust contaminated in this way catalyzes the formation of more rust. Direct exposure of a metal to rain may, therefore, be benefi cial, compared to a partially sheltered exposure. This advan- tage presumably would not extend to uncontaminated atmospheres.