Page 181 - CW E-Magazine (8-10-2024)

Special Report

tions with pH values between 6 and 10, to corrosion than SS. These alloys are Bonded Epoxy (FBE) and PTFE are
CI, which contains about 4% carbon, generally known by their tradenames, available.
quickly acquires a protective graphite for example: Hasteloy, Incoloy, Inconel,
layer. In fact, poorer quality CI with Monel, etc. The effectiveness of a coating gene-
higher carbon content is more resistant rally depends on:
to corrosion than steel. Thermoplastics The type of coating;
Thermoplastics are resistant to most The preparation of the surface to be

Stainless steel (SS) liquids that corrode metals. They are coated;
SS contains a significant propor- characterised by an ability to be soft- The application of the coating;
tion of chromium (at least 15%), which ened by heating and cast into complex The type of corrosion involved; and

forms a thin, inert oxide layer on the shapes. It is therefore possible to mould The type of fluid the coating is

surface. Unlike iron oxide, this binds individual components or even an exposed to.
strongly to the metal surface and there- entire pump case. Mechanical strength
fore protects the underlying metal in can be reinforced with fillers such as How are pumps lined
environments that would be highly cor- glass fibre. However, physical proper- To apply a metallic coating, various
rosive for CI or carbon steel (CS). Other ties are generally inferior to those of techniques can be used:
elements such as nickel, manganese equivalent metal pumps. Thermal spray: a stream of molten
and molybdenum may also be added to particles, generated using an elec-
SS to further enhance corrosion resis- PTFE (Teflon is a common trade- tric-arc, plasma or a combustion
tance. Under the right conditions, the name) is used for many pump com- process, is projected onto the base
protective oxide layer is self-repairing ponents including gaskets, seals and material.
if it is scratched or damaged. diaphragms because of its excellent Spray & fuse: the coating is sprayed

corrosion resistance and chemical in liquid form onto the component,
There are four main types of SS inertness. Unlike most thermoplastics, which is subsequently heated to
based on their crystal structures: auste- PTFE also has a wide operating tem- 950°C or more to fuse the coating.
nitic, ferritic, martensitic and duplex. perature range (up to 250°C). Plasma transferred arc (PTA): a
The most common types of SS used in welding process with elements of
pump manufacture are the 300 series in Coating technology both thermal spray and spray & fuse
the austenitic family, particularly 304 Another method of dealing with techniques.
and 316. The 304 grade contains 18% corrosion is to apply a coating. A coat- Laser cladding: similar to PTA, but
chromium and 8-10% nickel. The 316 ing isolates the metal surface from its using a laser as the heat source.
grade also includes 2% molybdenum environment. A wide range of factory-
and has greater resistance to acids and applied coatings, such as: Fusion Thermoplastics, such as PTFE, can
to localized corrosion.
Table. 1: Additional metallic coatings applied to pump components
In some cases, the use of CS or CI is for added corrosion resistance
preferred to SS for cost reasons. Corro- Major constituents Applications
sion is accepted, modelled reasonably Cobalt-chrome-tungsten alloys Good corrosion and wear resistance.
accurately, and factored into on-going (e.g., Stellite/Wallex)
maintenance and equipment replace- Nickel-based alloys (e.g., Colmoney) Good resistance to corrosion and heat.
ment costs. However, if the expected Nickel-chromium-molybdenum alloy Excellent resistance in oxidizing and reduc-
corrosion rate is high and seriously (Hastelloy) ing media, such as acids, even at elevated
limits service life, then SS pumps, temperatures. High resistance to cracking,
although more expensive, may be cost- pitting and stress corrosion.
effective in the long term. Aluminium oxide/titanium dioxide Range of ceramic powders applied by
(e.g., Amdry/Metco) thermal spray, resistant to wear and good
Superalloys thermal and chemical resistance.
For dealing with caustic media Chromium oxide Improves corrosion, abrasion and wear
such as acids and alkalis, particularly resistance.
at elevated temperatures, nickel-based Tungsten carbide (UCAR LW-15) High wear resistance (for applications
“superalloys” have a higher resistance below 500°C).

Chemical Weekly October 8, 2024 181

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