FlueGard-225SQC in 56% Sulfuric Acid: Six-Month Immersion Results

Sulfuric Acid Immersion Test: Key Points

• FlueGard®-225SQC on blasted carbon steel was immersed continuously in 56% sulfuric acid at 60 °C for six months, from 22 December 2006 to 22 June 2007.
• The test followed a NACE TM0174-91 basis and evaluated both the immersed portion and the vapor phase.
• After six months: thickness 20 mils and pencil hardness H retained, Shore D 85 unchanged, holiday test passed at 2.0 kV, electrochemical impedance spectroscopy (EIS) log modulus still above 9 at 0.1 Hz, knife-peel adhesion unchanged.
• This is Polylloy's own test, reviewed and approved by Edita Rojasova on 17 September 2025; it is not an independent test.
• One panel at one concentration and temperature is screening evidence, not a field service-life result; confirm the actual medium, temperature and exposure with Polylloy's technical team.
• FlueGard-225SQC is rated for immersion service and for flue gas to 225 °C (437 °F).

The FlueGard-225SQC sulfuric acid immersion test shows the coating passed six months of continuous immersion in 56% sulfuric acid at 60 °C without observed deterioration, in Polylloy's own NACE TM0174-91-based test run from December 2006 to June 2007. Thickness, hardness and adhesion were retained, the holiday test passed at 2.0 kV, and EIS stayed above log 9 at 0.1 Hz.

The test is Polylloy's six-month laboratory evaluation of the coating on carbon steel, with thickness, hardness, holiday, adhesion and electrochemical impedance checks on the immersed and vapor-phase areas. This page sets out the method, the results at each inspection and the limits of what one immersion test can show.

FlueGard-225SQC Acid Immersion Test: Engineer Questions vs What the Report Shows

Specifying engineers at engineering, procurement and construction (EPC) firms and original equipment manufacturers (OEMs) ask much the same questions before accepting acid immersion data into a specification. On "concentrated" acid, the honest answer is that 56% is the tested strength; stronger acid was not tested in this report.

Challenge vs Solution Table for Specifying Engineers

Each specification question below is matched with what the report shows and the limit that applies to it.

The limit column matters as much as the result column. A specification that cites this test should carry its conditions with it, so the data is not applied to a service it did not cover.

How Polylloy Ran the 56% Sulfuric Acid Immersion Test on FlueGard-225SQC

The method follows a recognised test basis and documents the panel, preparation and coating build. Every condition below comes from the test report.

Test Basis, Panel and Preparation

The test basis was NACE TM0174-91, using an Atlas-type cell. One area of the panel sits in the liquid (the immersed portion, IP) and one in the vapor above it (the vapor phase, VP).

The substrate was a sandblasted carbon steel plate, 8 × 8 × 1/8 in., prepared to NACE No. 2 near-white metal blast with a profile of about 3 mils (75 µm). The coating was air-sprayed in two coats to a total dry film thickness (DFT) of 20 mils, about 10 mils per coat, and the panel received a laboratory heat cure before exposure.

Hardness was measured as pencil hardness to ASTM D3363 and Shore D to ASTM D2240, and adhesion by knife to ASTM D6677. Visual inspection, holiday detection and EIS completed the assessment.

Exposure and Inspection Dates

The panel was exposed continuously to 56% sulfuric acid at 60 °C from 22 December 2006 to 22 June 2007. Inspections were made at 1 month (24 January 2007), 3 months (24 March 2007) and 6 months (22 June 2007).

FlueGard-225SQC Results at One, Three and Six Months

The report records no observed deterioration at any inspection. Where the report does not give a value at an inspection, the table says so rather than filling the gap.

Inspection Results Table

Why the Holiday Voltage Was 2.0 kV

The report applies a maximum of 100 V per mil under NACE TM0174-91, which at 20 mils gives 2,000 V (2.0 kV). Passing at the start and after six months means no through-film defects were detected in either the immersed portion or the vapor phase.

What the EIS and Adhesion Results Mean

EIS measures how strongly the film resists the passage of water, ions and dissolved gases. High impedance indicates a low-permeability barrier.

The six-month spectrum followed essentially the same response as the initial one, with log impedance modulus above 9 at 0.1 Hz. That means no detected loss of barrier performance over the exposure.

Knife-peel adhesion at six months was rated excellent in both IP and VP. There was no observable difference from the unexposed part of the plate.

What the FlueGard-225SQC Immersion Results Do and Do Not Prove

The results show retained barrier performance under one set of laboratory conditions. They do not, on their own, predict how long the coating will last in a given plant.

Scope of the Evidence

• Who ran it: this is Polylloy's own test, reviewed and approved by Edita Rojasova, Co-founder and Chief Technology Officer, on 17 September 2025. The data dates from 2006–07, and it is not an independent or third-party test.
• What was tested: one coated plate, one acid strength (56%), one temperature (60 °C), continuous immersion. Stronger acid, other acids and higher temperatures were not tested in this report.
• How it was cured: the panel was heat-cured in the laboratory. In the field, final heat cured activation is completed during start-up using process heat, so field cure quality and surface preparation need their own inspection.
• What was absent: no thermal cycling, abrasion or deposit chemistry. It is a screening result, not a field service-life claim.

How to Use It Alongside Other Data

Pair the six-month result with the FlueGard-225SQC bulletin's separate one-week immersion weight-gain data: 37% hydrochloric acid below 1.0%, 54% phosphoric acid below 0.5%, 36% sulfuric acid below 0.5% and 3% hydrofluoric acid below 1.0%. Field evidence is a different kind of proof, set out in the coal baghouse coating service record.

Where FlueGard-225SQC Sulfuric Acid Resistance Matters in Flue Gas Equipment

Sulfuric acid forms where flue gas containing sulfur trioxide (SO3) and moisture cools below its acid dew point on steel. That happens in baghouses, electrostatic precipitators (ESPs), ducts, stacks and flue gas desulfurization (FGD) scrubbers, all listed applications for FlueGard-225SQC.

Equipment and Exposure

FlueGard-225SQC is a two-component polymeric alloy, 100% solids, rated for flue gas to 225 °C (437 °F) and for immersion service. Its bulletin lists resistance to SO3, sulfur dioxide (SO2), hydrogen chloride (HCl), hydrogen fluoride (HF) and nitrogen oxides (NOx) gases. Product details are on the FlueGard acid-resistant coatings page.

Condensate strength and temperature vary by plant, so the 56% test strength should not be read as typical condensate. Alloys suffer in the same conditions, as explained in why 316 stainless corrodes in flue gas, and wet zones are covered in flue gas desulfurization corrosion protection.

Other lining chemistries are also used against acid condensate. The differences are set out in the comparison of novolac, vinyl ester and hybrid coatings.

Deciding Whether FlueGard-225SQC Immersion Data Fits Your Specification

When comparing any supplier's acid immersion data, check that the report states:

• Method standard
• Substrate and surface preparation
• DFT
• Cure route
• Acid, strength and temperature
• Duration
• Immersed and vapor phases
• Holiday voltage
• EIS or another barrier measurement
• Who ran and who reviewed the test

If your medium, strength or temperature differs from 56% at 60 °C, request project-specific confirmation rather than extrapolating. To request a coating evaluation, share equipment type, coated area, operating conditions (temperature, gas chemistry or immersion medium, abrasion), location and outage timing.

FAQs

Has Any Coating Been Tested in Concentrated Sulphuric Acid?

FlueGard-225SQC has been tested in 56% sulfuric acid at 60 °C for six months of continuous immersion, with no observed deterioration. Thickness, hardness and adhesion were retained and the holiday test passed at 2.0 kV. Stronger acid, such as above 90%, was not part of this test, so confirm suitability for higher strengths with Polylloy's technical team.

What Standard Was Used for the FlueGard-225SQC Acid Immersion Test?

The test was run on a NACE TM0174-91 basis, which exposes a coated panel so that one area is immersed and another sits in the vapor phase. Pencil hardness followed ASTM D3363, Shore D hardness ASTM D2240 and knife adhesion ASTM D6677. Holiday detection, visual inspection and electrochemical impedance spectroscopy completed the assessment.

Is the FlueGard-225SQC Sulfuric Acid Test Independent?

No. It is Polylloy's own laboratory test, run from December 2006 to June 2007, and the report was reviewed and approved by Edita Rojasova, Co-founder and Chief Technology Officer, on 17 September 2025. Engineers who need third-party data should ask Polylloy what is available for their medium or specify an independent test for the project.

What Does an EIS Log Modulus Above 9 Mean for a Coating?

Electrochemical impedance spectroscopy measures how strongly a coating resists the flow of current carried by water and ions through the film. A log impedance modulus above 9 at 0.1 Hz indicates a high-resistance, low-permeability barrier. For FlueGard-225SQC, the value stayed above 9 after six months in 56% sulfuric acid, showing no detected barrier loss.

Does a Six-Month Immersion Test Predict Coating Service Life?

No. An immersion test screens a coating's chemical resistance under fixed laboratory conditions, while field life also depends on temperature cycling, abrasion, deposits, surface preparation, film build and cure quality. The FlueGard-225SQC result shows retained barrier performance in 56% sulfuric acid at 60 °C; field service records are a separate kind of evidence.

Is FlueGard-225SQC Rated for Immersion Service?

Yes. Polylloy's brochure lists FlueGard-225SQC as rated for immersion, and its bulletin reports one-week immersion weight gain below 0.5% in 36% sulfuric acid and 54% phosphoric acid. Suitability for a specific tank, sump or wetted zone still depends on the chemical, strength and temperature, so confirm the duty with Polylloy before specifying.