13-08-2012, 05:08 PM
A REPORT ON THE PERFORMANCE OF RUST ELIMINATORS NR-661 AND NR-661 HB ON MILD STEEL
INTRODUCTION
In recent years, interest has been shown in the development of coatings
for the steels which could provide a corrosion resistant surface by interacting
with corrosion product(s) of the steel [1,2]. There are variety of mechanisms
by which these coatings work; they can be classified as the products that
impregnate rust, convert rust to magnetite, inactivate soluble salts or convert
iron oxides to other products. The last category is known as rust conversion
surface coatings.
Rust conversion coatings are promoted as water-based products reacting
directly with a rusted surface to form an inert, water insoluble complex that
can be top coated. The vast majority of commercial rust converters incorporate
some type of polyhydroxylated or tannin like compound. The transformation
of a rusty surface to the blue-black coating which occurs after the product’s
application has been attributed to the complexation of tennin resin resulting in
the formations of iron oxide and hydroxide in rust and a ferric-tennate complex
[3,4].
NR-661 and NR-661 HB are the Commercial Rust Eliminators developed
by INCORP for applications on rusted iron and steel surfaces [5]. They are
off-white miliy liquids (Sp gravity : 1.25 - 1.26 g/cc) and are organic anti-oxidants
in a modified vinyl acrylic copolymer emulsion. No informations about the
chemical compositions or molecular structures of the two rust eliminators is
provided by the manufacturers. NR-661 on applying to rusted ferrous metal
surfaces, chemically converts the rust surface (ferric hydroxide) into a hard inert
neutral surface (fersoferric oxide) which is a fire retardant. According to the
manufacturer, the hard black surface forms an impermeable barrier to oxygen
and moisture and will withstand temperature in the range 40 to 150 C.
EXPERIMENTAL
Mild Steel (0.25% C) specimens of dimensions 2.5x1.5x0.15 cm were cut
from sheets and were abraded sequentially with 180, 320, 400 and 600 grade
SiC papers. The coupons were kept in moist environments for 24 hours to get a uniform layer of rust of about 100 microns ( 4 mils). A thin coating of rust
eliminator was applied on the rusted surface using a brush. For subsequent
double or triple coatings, 24 hours gap is given after each coating. On applying
rust eliminator, the rusted surface usually becomes deep blue or black.
Two types of tests were performed while evaluating the performance of
rust eliminator. In type I, the weight changes in coated specimens were determined
gravimetrically with increasing exposure time. In type II, the effect of coating
thickness on the corrosion rate was determined.
RESULTS
The coating evaluation tests on NR-661 and NR-661 HB coated steel
were carried out by exposing the specimens to moisture, seawater and salt spray.
The weight changes were recorded at a regular interval of 24 hours or its multiple
and the test runs were usually of 720 hours durations. The influence of environment
and coating thickness on the corrosion rate of coated steel was studied.
Salt Spray Tests
Figure 1 shows plots of weight gain vs time for single, double and triple
coatings of NR-661 rust eliminator, respec- tively. The coated steel specimens
were exposed in a salt spray chamber according to ASTM B117-73 specifications.
The NR-661 single coated steel specimen shows a steep rise in weight gain values
with increasing exposure time. Infact an approximate linear relationship exists
between weight gain and exposure time (Figure la). In double and triple coatings
the weight gains are much lower than the single coatings. The double and triple
NR-661 coated steel show a maximum weight gain after 660 hours followed by
a decrease in weight gain values.