Page 175 - CW E-Magazine (2-4-2024)

Special Report

corrosion failure due to fabrication and mixed with long-life materials in non- The following should be noted with
assembly processes such as welding, repairable subassemblies. Materials regard to electrical equipment. The use
forming, machining, heat treatment, forming thick scale should not be used of hygroscopic materials and desic-
etc., are of prime importance for selec- where heat transfer is important. cants should be avoided. The later,
tion of material and design. when their use is necessary, should
Where materials could be exposed not be in contact with an unprotected
Due consideration should be given to atomic radiation it is necessary to metallic part. Fasteners should be of a
to special treatments required to consider whether the effect will be well-selected corrosion resistant mate-
improve resistance to corrosion, for derogatory or beneficial, observing that rial, or materials better protected than
example- special welding techniques, some controlled radiation may enhance the parts that they join together. Insula-
stress relieving, blast peening, metallising, the property of a metal. tion material should not be susceptible
sealing of welds, etc. Also, to any fab- to moisture. SS or precipitation harden-
rication or assembly methods, which Not only the structural materials ing SS should be passivated.
would aggravate any tendency of the themselves, but also their basic treat-
material to corrosion failure. ment should be evaluated for suitability Assessing the cost of materials
(e.g., chromate passivation, cadmium selection
It is sometimes better to use a some- plating, etc.) at the same time. Cost is of major consideration for
what weaker but less sensitive alloy, materials selection, and the balance
than to use the one which does not lend Non-metallic materials complying of capital expenditures (CAPEX) and
itself to reliable heat treatment and, due with the following requirements are operating expenses (OPEX) is cru-
to this, whose resistance to a particular preferred: no moisture absorption, cial. Materials and corrosion engineers
corrosion is poor. resistance to fungi and microbes, stability should select materials that satisfy the
throughout temperature range, compati- considerations of budget and costs, as
Materials prone to SCC should be bility with other materials, resistance to well as performance. The significance
avoided in environments conducive to flame and arc, freedom from out-gassing of CAPEX lies not only in the bulk
failure, as stress relieving alone is not and ability to withstand weathering. materials selected, but also in the fabri-
always a reliable cure. cation processes and product forms. The
Flammable materials should not be impact of OPEX lies in the repair and
When corrosion or erosion is used in critical places. There heat could replacement of corroded materials, and
expected, an increase in wall thickness of affect corrosion stability of structural the corrosion control, corrosion moni-
the structure or piping should be provi- materials. Toxic materials producing toring and corrosion inhibitor injection.
ded over that required by other functio- dangerous volumes of toxic or corrosive Using carbon steel might be of low
nal design requirements. This allowance gasses when under fire or high tempera- CAPEX, but of high OPEX, and using
in the judgement of the designer should ture conditions should not be used. SS impacts each in the opposite way.
be consistent with the expected life of
the structure or piping. The allowance Fragile or brittle materials which Materials availability also plays a
should secure that various types of cor- are not, by design, protected against crucial role, especially regarding pro-
rosion or erosion (including pitting) do fracture should not be used in corro- ject schedule. Use of proprietary mate-
not reduce the thickness of the structure sion-prone spaces. rials, for example, might take longer
or piping below the thickness that is than usual in delivery because of limi-
needed for mechanical stability of the Materials that produce corrosion tations related to the suppliers, low
product. Where no thickening can be products that can have an adverse quantities, or stocks that could require
allowed or where lightening of pro- effect on the quality of contents should minimum quantity orders, in situations
duct is contemplated a proportionally not be used, especially when the cost of that could increase cost.
more resistant alloy or better protection wasted contents exceeds the cost of the
should be used. In general, and subject container. Difficult-to-fabricate materials might
to mechanical requirement, wall thick- significantly impact the project
ness is made double the thickness that All effort should be made to obtain schedule if not planned early, espe-
would give desired life. from the suppliers of the equipment an cially when used in large quantities.
accurate detailed description of materi- For example, materials that need post-
Short-life materials should not be als used within their products. weld heat treatment (PWHT) might

Chemical Weekly April 2, 2024 175

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