Page 169 - CW E-Magazine (2-4-2024)

Special Report

grades). Their strength can be higher assisted SCC. There are many types of 250Å in four years. Unlike some metals
than that of martensitic SS due to the nickel alloys that are designated by brand whose oxide layer can be flaky, brittle,
addition of niobium, copper, and alu- name, such as Hastelloy, Incoloy, Monel, and eventually spall off (such as steel),
minium up to 0.5%. Chromium addi- etc. These names pertain to expired pat- titanium’s tenacious oxide layer acts as
tion is to the tune of 15-17.5%. Due to ents, but the naming legacy remains. a barrier that prevents corrosive agents
their mechanical strength these grades from coming into contact with the
are the preferred materials for long Aluminium alloys underlying metal substrate. This layer
shafts. Precipitation hardened SS have Aluminium alloys have good becomes stronger and more resilient over
high strength and toughness, from aus- strength-to-weight ratio and good cor- time and can regenerate almost imme-
tenitic, semi-austenitic, or martensitic rosion resistance. They are, however, diately if broken. In oxidising condi-
microstructures, depending on the heat susceptible to galvanic corrosion when tions, such as aqueous environments,
treatment. Although they have good in contact with iron-based metals. As the film consists mainly of TiO (tita-
2
weldability, their applications are limi- aluminium has a low melting tem- nium dioxide). TiO (titanium oxide)
ted, such as for high-performance perature, the application of aluminium is also another variation of the surface
springs. alloys is limited to low-temperature oxide film. In high-temperature oxidising
uses. Because of their excellent thermal situations, the chemically resistant and
Alloys performance and ductility retention, highly crystalline form of TiO , known
2
An alloy combines different metals, aluminium alloys are used in cryogenic as rutile, is formed. Conversely, oxida-
allowing for accommodating different applications, such as heat exchangers in tion at lower temperatures can produce
properties. Here are some of the most the LNG industry. the more amorphous structure of TiO
2
common alloys and their key characte- known as anatase. Other environmental
ristics. Copper alloys conditions may also result in the for-
Copper alloys are commonly used mation of Ti O .
3
2
Nickel alloys in seawater environments and heat ex-
Nickel and its alloys have excep- changers. They have high resistance to Although titanium performs well
tionally good atmospheric corrosion bio-fouling, especially micro-fouling, in most corrosive environments, it
resistance. The corrosion rates are typi- and stagnant conditions that can cause can be susceptible to degradation
cally less than 0.0025-mm/yr with vary- localised corrosion, as well as corro- under certain conditions. The corrosion
ing degree of surface deterioration, but sion related to excessive flow. Some resistance of titanium can be severely
they are expensive because of their high copper alloys are susceptible to SCC in affected in anhydrous environments,
nickel content, which is usually added environments containing ammonia. i.e., environments containing little to
in combination with other alloying no water. Titanium derives its corrosion
elements. The most common wrought Titanium (Ti) alloys resistance from its oxide film that forms
corrosion resistant Ni-Cr metals are Ti has many desirable physical proper- in the presence of moisture and oxygen.
Inconel 600 and Inconel 625. The pri- ties including outstanding corrosion Generally, even trace amounts of mois-
mary corrosion resistance attributes of resistance. However, this metal is not ture can be extremely beneficial in passi-
alloy 600 include excellent resistance corrosion-proof, and the limitations of vating the titanium surface. However,
to caustic solutions and good resistance its tenacity should be fully understood. in anhydrous media, the oxide film may
to SCC relative to many SS. Ni can be Ti alloys have good strength-to-weight not form, or if it does, it will be diffi-
alloyed with Cr, Cu, and Mo for cor- ratio and high corrosion resistance in cult to sustain. Methanol, for example,
rosion resistance and for retaining its high chloride environments. This is due can result in SCC in unalloyed titanium
ductile FCC structure. Cr enhances the to the stable, continuous and tightly at moisture contents below 1.5%.
resistance of Ni to oxidising acids by adhered protective oxide film that forms Hydrogen embrittlement has also been
encouraging formation of passive films. almost immediately on the metal’s sur- observed in high-temperature anhydrous
Cu is very helpful in seawater, brackish face when exposed to air and moisture. conditions. Other environments known
water, and reducing agents (particularly At room temperature, after a clean to promote SCC are those containing
hydrofluoric). Mo is extremely bene- titanium surface is exposed to air and red fuming nitric acid, nitrogen tetr-
ficial in all reducing agents. moisture, the oxide film produced is oxide and gaseous bromine and fluorine.
about 12-16Å thick. After 70 days, the
Nickel alloys are also resistant to film grows steadily to 50Å, slowly in- Coupling titanium with a more
chloride-assisted SCC and sulphide- creasing to 80-90Å after 545 days, and noble metal in an electrolyte does not

Chemical Weekly April 2, 2024 169

Contents Index to Advertisers Index to Products Advertised