FRP vs PP vs GI Scrubbers: Material Selection Guide for Corrosive Gas Applications

FRP vs PP vs GI scrubber material selection comes down to matching the vessel’s material to the specific chemical, temperature, and structural demands of the gas stream it will handle, not defaulting to whichever material is cheapest or most familiar. Getting this choice wrong doesn’t usually cause an immediate failure; it shows up months or years later as premature corrosion, leaks, or structural degradation in a vessel that looked adequate on day one, often long after the original purchasing decision has been forgotten and the cost of correcting it falls on whoever is running the facility at the time.

The Three Materials at a Glance

Material	Chemical resistance	Structural strength	Relative cost	Typical role
FRP (Fibre-Reinforced Plastic)	Broad resistance to acids and alkalis, tunable by resin selection	Good, especially for large self-supporting vessels	Moderate to higher for large vessels	Primary construction material for most corrosive-duty scrubbers
PP (Polypropylene)	Strong resistance to many acids, especially at moderate concentrations	Lower rigidity than FRP at large scale; often needs external support	Lower material cost, but often combined with FRP for structure	Wetted lining/liner material, frequently paired with an FRP outer shell
GI (Galvanized Iron)	Limited — zinc coating is attacked by acidic and many alkaline fume environments	High structural strength, low cost	Lowest	Non-wetted structural components, ducting for low-corrosivity or dilution air, not typically the scrubbing chamber itself in aggressive service

FRP: The Default for Broad Corrosive Duty

FRP is fabricated by combining a resin system (commonly vinyl ester or isophthalic polyester, chosen based on the specific chemical exposure) with glass fibre reinforcement, giving a vessel that is both chemically resistant and structurally self-supporting even at large scale. This combination is why FRP is the default construction material for most custom-fabricated wet scrubbers handling acid gases, alkaline fumes, or mixed corrosive streams — see our FRP scrubber manufacturing overview.

Resin selection matters as much as the fact that it’s “FRP” — a vinyl ester resin system generally offers broader chemical and temperature resistance than a standard isophthalic polyester system, at a higher cost. Specifying “FRP” without confirming the resin system is like specifying “steel” without confirming the grade — the base material category doesn’t guarantee the specific chemical resistance a given application needs.

PP: Lightweight Chemical Resistance, Often Paired With FRP

PP offers strong resistance to many acids, particularly at moderate concentrations and temperatures, and is lighter and often less costly per unit area than FRP for smaller components. Its limitation is structural: PP alone becomes less rigid at larger vessel sizes and higher temperatures, which is why it’s frequently used as an internal liner material bonded to or backed by an FRP or other structural outer shell rather than as the sole material of a large freestanding vessel — a combination reflected in product lines like PP-FRP scrubbers, which pair PP’s chemical resistance at the wetted surface with FRP’s structural rigidity in the outer shell.

PP’s practical temperature ceiling is generally lower than FRP’s (particularly lower than a high-performance vinyl ester FRP system), which matters for hot process gas streams — a scrubber handling gas at the upper end of PP’s temperature tolerance may need either a cooling stage upstream or a switch to a higher-temperature-rated material.

GI: Structural Strength Without Corrosion Resistance

Galvanized iron offers good structural strength at low cost, but its zinc coating provides only limited protection against acidic or alkaline atmospheres — exactly the environment inside a wet scrubber handling corrosive gas. This is why GI is generally reserved for structural framework, support steelwork, and ducting carrying non-corrosive or already-scrubbed (clean) air, rather than for the scrubbing chamber itself in an aggressive chemical duty. Where a project’s ducting run includes both a corrosive section (upstream of the scrubber) and a clean-air section (downstream, after scrubbing), it’s common and appropriate to specify different materials for each section rather than using one material throughout — see our GI ducting and FRP ducting pages for where each is typically appropriate.

Matching Material to Specific Gas Streams

“Corrosive gas” isn’t a single design condition — different gases attack materials differently, and a material well-suited to one chemical exposure isn’t automatically suited to another:

• Ammonia (NH3): Requires a material and resin system specifically compatible with alkaline conditions, since ammonia scrubbing typically operates in an alkaline or near-neutral regime rather than the strongly acidic conditions many acid-gas scrubber materials are optimized for. See our ammonia scrubber page for application-specific detail.
• Chlorine and chlorine-derived acids: Highly aggressive to many common metals and to some resin systems; material selection here leans toward chlorine-resistant resin grades and, in some designs, PP linings specifically chosen for their chlorine resistance profile. See our chlorine gas scrubber page.
• Sulfur dioxide / sulfuric acid mist: Common in flue gas desulfurization and similar applications, generally requiring a resin system rated for sustained sulfuric acid exposure across the operating temperature range. See our SO2 scrubber page.
• Mixed or variable acid fume streams (common in electroplating and general chemical processing) often justify a more broadly resistant vinyl ester FRP system specifically because the exposure isn’t a single, well-characterized chemical, making a narrower-spectrum material a riskier choice.

The practical implication is that material selection should start from the actual process chemistry, not from a generic “acid-resistant” specification — the same base material category can perform very differently against ammonia versus chlorine versus sulfuric acid mist.

Maintenance and Inspection Differences by Material

Material choice also affects what an inspection program should look for, which is worth knowing before the vessel is in service rather than after a problem develops:

• FRP vessels should be inspected for surface cracking, delamination, and resin-rich or resin-starved areas (visible as color or texture variation), particularly at joints, nozzle penetrations, and other high-stress points where the laminate is most likely to be thinner or more stressed than the flat vessel wall.
• PP linings should be checked for softening, deformation, or stress cracking, especially near the upper end of PP’s temperature tolerance, and for proper bond integrity where the liner meets its structural backing.
• GI structural components in or near the corrosive zone should be checked for zinc coating degradation and base metal corrosion at a more frequent interval than GI used purely for clean-air ducting elsewhere in the system, precisely because it’s operating closer to (or arguably beyond) its appropriate service envelope.

Selection Factors Beyond the Base Material

• Specific chemical exposure: The exact gas being scrubbed (ammonia, chlorine, sulfur dioxide, hydrochloric acid fume, mixed acid fumes) determines which resin system or material grade is actually appropriate — a material that resists one acid well may perform poorly against another, so selection should be based on the specific chemical, not a generic “corrosive gas” assumption.
• Concentration and temperature together: Chemical resistance data is typically only valid within a stated concentration and temperature range — a material rated resistant to a chemical at room temperature may degrade faster at elevated temperature, even at the same concentration.
• Vessel size and structural demand: Larger vessels place more structural demand on the material, which is part of why PP is more often paired with FRP as vessel size increases, rather than used alone.
• UV and outdoor exposure: FRP and PP surfaces exposed to direct sunlight over years can experience UV-related surface degradation if not specified with appropriate UV-resistant additives or a protective gel coat, which matters for outdoor-installed scrubbers more than indoor ones.
• Mechanical/abrasion demand: A gas stream carrying entrained particulate alongside corrosive gas adds an abrasion consideration on top of pure chemical resistance, which can favor a thicker laminate or a different internal design regardless of which base material is chosen.

Cost Considerations Across the Vessel’s Service Life

Purchase price is only one part of the real cost comparison between materials. A lower-cost material that fails prematurely against its actual chemical exposure creates replacement cost, unplanned downtime, and potentially a compliance risk if the scrubber is a permitted emission control point — costs that a higher initial-cost, properly-matched material avoids over its longer service life. This doesn’t mean the most chemically resistant, highest-grade material is always the right economic choice either — a vinyl ester FRP system costs more than a standard polyester system, and that premium is only justified where the actual chemical exposure requires it. The right comparison weighs expected service life against the specific application’s chemical severity, not purchase price in isolation, and not “the most resistant material available” as a default without regard to whether that resistance is actually needed.

Structural Design Considerations Alongside Material Choice

Material selection interacts with vessel design in ways that go beyond simple chemical compatibility. A larger-diameter vessel places more hoop stress on its walls at a given internal pressure, which affects the required laminate thickness for FRP or the support structure needed for a PP-lined design — meaning the same material choice can require a meaningfully different specification at a larger scale even for an identical gas stream. Nozzle penetrations, internal packing support structures, and mist eliminator mounting points are all locations where the vessel wall is locally more stressed than the general shell, and where inadequate reinforcement — regardless of which base material is used — is a common source of premature failure that isn’t really a “wrong material” problem so much as an inadequate structural design problem layered on top of an otherwise reasonable material choice.

Working With a Fabricator’s Chemical Resistance Data

Reputable FRP and PP fabricators maintain chemical resistance charts and data sheets covering specific resin and material grades against a range of chemicals, concentrations, and temperatures. Reviewing this data against the actual, specific process conditions — not a rough approximation of them — is the practical way most real projects arrive at a material specification, rather than deriving resistance from first-principles chemistry. Where the process gas composition is genuinely uncertain or variable (common in some batch chemical processes), it’s worth specifying material resistance against the worst reasonably expected condition rather than the typical operating condition, since a scrubber that performs adequately most of the time but degrades rapidly during occasional worst-case excursions still represents a real long-term reliability risk.

Common Material Selection Mistakes

• Specifying “FRP” without confirming the resin system, assuming all FRP offers equivalent chemical resistance regardless of the specific resin used in fabrication.
• Using GI for the wetted scrubbing chamber in genuinely corrosive service, rather than reserving it for structural or clean-air-side components.
• Ignoring temperature in the material selection, checking only chemical compatibility at an assumed room temperature when the actual process gas runs hotter.
• Assuming one material choice fits the entire ducting and scrubber system, when different sections (corrosive inlet duct, scrubbing vessel, clean outlet duct) may reasonably use different materials matched to their specific exposure.
• Not accounting for outdoor UV exposure in the material or coating specification for scrubbers installed without a sheltering structure.
• Sizing the material specification against typical operating conditions only, without checking resistance against the worst reasonably expected excursion in concentration or temperature that the process could produce.
• Under-reinforcing high-stress points like nozzle penetrations and internal support structures, treating material selection as the only variable that matters while overlooking localized structural design at these connection points.

When to Involve a Materials or Process Engineer

Straightforward, well-characterized applications — a single known gas at stable concentration and temperature — can often be specified confidently using a fabricator’s standard chemical resistance data. Applications with mixed or variable gas streams, elevated temperatures near a material’s practical limit, unusual concentrations, or any combination of corrosive gas with entrained particulate causing simultaneous chemical and abrasive attack are better served by involving a materials or process engineer early in the specification, rather than defaulting to a standard product line and hoping it holds up. The cost of that additional engineering input is small relative to the cost of a scrubber vessel that fails prematurely in service, particularly where that scrubber is the facility’s primary control for a regulated emission point.

Frequently Asked Questions

Is FRP always better than PP for a scrubber?

Not universally — FRP generally offers better standalone structural rigidity at large scale, while PP offers strong chemical resistance at lower material cost for smaller components or as a liner. Many applications combine both, using PP for the chemically-exposed inner surface and FRP for structural support, rather than treating the choice as either-or. The right answer depends on vessel size, the specific chemical exposure, and budget, more than a general preference for one material over the other.

Can GI be used anywhere in a scrubber system?

GI is generally appropriate for structural framework and for ducting carrying non-corrosive or already-scrubbed clean air, but not for the scrubbing chamber itself or ducting carrying untreated corrosive gas, since its zinc coating doesn’t hold up well against sustained acidic or alkaline exposure. Where GI is used near a corrosive zone for structural purposes, it should be inspected more frequently than GI used purely for clean-air duty elsewhere in the system.

Does the resin type in FRP actually matter for scrubber performance?

Yes, significantly. Vinyl ester resin systems generally offer broader chemical and temperature resistance than standard isophthalic polyester systems, at a higher material cost. The specific resin should be selected against the actual gas being scrubbed and its concentration and temperature, not assumed from the general “FRP” material category.

Why do some scrubbers use PP-FRP construction instead of either material alone?

PP-FRP construction combines PP’s strong chemical resistance at the wetted surface with FRP’s structural rigidity in an outer shell, which is particularly useful at larger vessel sizes where PP alone would need additional structural support anyway. This gives the chemical resistance benefit of PP without the structural limitations of using it as a standalone material at scale, while generally costing less than an equivalent all-vinyl-ester FRP vessel would.

Does outdoor installation change the material choice?

It changes the specification within a material choice more than the base material itself — FRP and PP scrubbers installed outdoors typically need UV-resistant additives or a protective gel coat to prevent surface degradation from sustained sunlight exposure, which an indoor-installed unit of the same material may not require.

How do I know which resin grade my application actually needs?

Reputable FRP and PP fabricators publish chemical resistance data covering specific resin grades against a range of chemicals, concentrations, and temperatures. The practical approach is to check this data against your actual process conditions — including the worst reasonably expected excursion in concentration or temperature, not just typical operating conditions — rather than assuming a general “corrosion-resistant” specification is automatically adequate for a specific gas stream.

Getting scrubber material selection right the first time — against the actual gas, concentration, and temperature the vessel will see — avoids a costly mid-life replacement or retrofit. Envigaurd’s scrubber manufacturing team specifies FRP, PP, PP-FRP, and GI construction based on the actual gas stream, concentration, temperature, and structural requirements of each project. Talk to Envigaurd’s engineers about material selection before finalizing your scrubber specification.