5 Ways to Protect Existing 316 Stainless Steel Equipment from Acid Corrosion

Protecting Existing 316 Stainless Steel: Key Points

• Equipment already built in 316 stainless steel can be coated where the surface can be abrasive blasted to the grade's preparation standard and the grade suits the temperature and chemistry.
• Stainless needs the same near-white preparation and surface profile as carbon steel: SSPC-SP10 near-white with a profile above 3 mils (75 µm).
• Five ways cover most existing 316 equipment: a FlueGard® coating, removing cold spots and air in-leakage, controlling deposits and shutdown condensate, restoring welds and crevices, and replacing failed sections.
• At a cement plant coal baghouse in Togo, FlueGard-225SQC was applied in June 2025 to stainless-steel perforated plates alongside carbon-steel framing, and the coated surfaces were reported in perfect condition at the 2026 one-year inspection.
• Cause-side fixes (cold spots, deposits, welds) reduce attack but put no barrier on the steel; a practical plan combines one of them with a coating or targeted replacement.
• 316 that shows no attack in service needs no coating; mild service is not a fit for a severe-service coating.

Equipment already built in 316 stainless steel can be coated: blasted to the same near-white standard as carbon steel, it can take a FlueGard grade matched to its temperature and chemistry. Four other ways protect it too: removing cold spots, controlling deposits and shutdown condensate, restoring welds and crevices, and replacing failed sections.

Protecting Existing 316 Stainless Steel Equipment Explained

Protecting existing 316 stainless steel equipment means stopping or slowing acid and chloride attack on stainless baghouses, ducts, stacks and scrubber parts that are already built and in service. It is done by adding a barrier coating, changing the conditions at the metal surface, repairing vulnerable details or replacing sections that have failed.

Why Existing 316 Equipment Needs a Plan

316 relies on a thin chromium oxide passive film. Chloride ions break it locally and acidic condensate below the acid dew point stops it healing, causing pitting, crevice corrosion under deposits and chloride stress corrosion cracking near welds, as set out in why 316 stainless corrodes in flue gas.

Thin metal leaves little margin on existing equipment. For a nominal 1-2 mm 316L liner, Polylloy's technical paper on stainless steels notes that local penetration rather than average thinning may govern service life. For new builds rather than equipment in service, the full set of alternatives to 316 stainless steel compares the material options.

How the 5 Ways to Protect Existing 316 Stainless Steel Were Selected

The five ways were chosen for protecting in-service 316 equipment from acid corrosion.

Selection Criteria

Each way was assessed against five criteria:

• It works on equipment already built, not only at design.
• It addresses acid or chloride attack on 316 specifically.
• It is clear whether it puts a barrier on the steel or reduces exposure.
• It can be planned into an outage or done while running.
• Its limits are stated honestly, including when 316 needs nothing.

Order follows fit to that duty: the FlueGard coating first, because it isolates the existing stainless without replacing it, then cause-side measures, then replacement for metal past repair.

Five Ways to Protect Existing 316 Stainless Steel at a Glance

The table compares the five ways side by side.

Coating Existing 316 Stainless Steel with FlueGard from Polylloy Coatings

A FlueGard coating is a barrier that isolates the stainless from acidic chloride condensate, so the passive film no longer has to survive the condensate.

Suited To

In-service stainless baghouses, ducts, stacks and flue gas desulfurization (FGD) gas-side components with pitting or crevice attack, where the surface can be blasted and inspected and temperatures sit inside a grade's service limit.

What It Is

Polylloy Coatings matches grades by service limit. FlueGard-225SQC is a two-component polymeric alloy rated to 225 °C (437 °F), resistant to sulfur trioxide (SO3), sulfur dioxide (SO2), hydrogen chloride (HCl), carbon dioxide (CO2), hydrogen fluoride (HF) and nitrogen oxides (NOx), and immersion-rated. Its bulletin lists baghouses, electrostatic precipitators (ESPs), ducts and FGD scrubbers.

StackGard®-255SQW, rated to 255 °C (491 °F), covers steel chimneys, ducts and fans. FlueGard-425S, rated to 425 °C (797 °F) with peaks to 500 °C, and FlueGard-455CHT, a ceramic rated to 455 °C (850 °F), cover hotter ducts.

Stainless needs the same near-white preparation and surface profile as carbon steel: SSPC-SP10, a Society for Protective Coatings (SSPC) standard, with a profile above 3 mils (75 µm). Condition, access and the outage window decide what is practical, and Polylloy's technical team confirms grade and preparation for each component; sequencing is covered in coating outage planning steps.

Evidence

At a cement plant in Togo, FlueGard-225SQC was applied in a supervised June 2025 campaign to about 875 m² of the main coal baghouse, a mix of carbon-steel framing and stainless-steel perforated plates, blasted to a minimum 75 µm profile. The coated surfaces were reported in perfect condition at the 2026 inspection, about one year later.

At Woodland Biomass Power Plant (Xcel Energy) in California, earlier protection of the baghouse clean-air section had been unsuccessful, and a FlueGard-225 trial compartment was reported in excellent condition with no corrosion after seven months. Results are project-specific.

Partners apply the coating; Polylloy formulates, supplies, specifies, trains and supervises. Final heat cured activation is completed during start-up using process heat. Grade data is on the FlueGard flue gas coatings page.

Pros and Cons

Pros

• Protects the existing stainless without replacing it.
• One barrier covers pits, crevices and weld zones on the coated surface.
• Grade ratings of 225 °C, 255 °C, 425 °C and 455 °C cover a wide temperature range.

Cons

• Needs abrasive blast access and an outage; heat-cured grades need start-up heat; any damage must be repaired.
• Parts that cannot be blasted or inspected, such as small-bore piping, bellows and tight internals, are not candidates.
• Existing cracks must be assessed and repaired first, not coated over.

Removing Cold Spots and Air In-Leakage on Existing 316 Equipment

Removing cold spots raises metal temperature where condensate forms.

Suited To

316 equipment whose damage clusters at doors, expansion joints, dampers, hopper walls, stiffeners or uninsulated areas.

What It Is

Survey skin temperatures with thermography and contact probes at normal load, low load and start-up, marking areas below the sulfuric acid and HCl dew points. Then seal air in-leakage, repair insulation, add heat tracing and improve start-up and shutdown purges.

On existing 316, comparing the damage map with the thermography map shows whether cold spots drive the attack and where to coat or repair first. Typical symptoms are listed in acid dew point corrosion warning signs.

Pros and Cons

Pros

• Treats the cause during operation, often with low intrusion.

Cons

• Cannot fully prevent shutdown condensate on deposits, and puts no barrier on the steel.
• Air leaks return unless seals are maintained.

Controlling Deposits and Shutdown Condensate on 316 Stainless Steel Surfaces

Deposit control targets the wet phase that forms when equipment cools.

Suited To

316 equipment that cycles often or stands for outages with dust and ash on its surfaces, especially where damage is found after shutdowns.

What It Is

Polylloy's technical paper describes the sequence: deposition in operation, cooling at shutdown, moisture uptake or condensation, salt dissolution, then localized corrosion beneath retained deposits. Dry operation does not prove shutdown exposure is benign.

Sample deposits and condensate for pH, chloride, sulfate and fluoride, and clean deposits before long outages where practical. Keep drains clear and log shutdown frequency alongside thickness data.

Pros and Cons

Pros

• Targets the crevice and under-deposit attack 316 is most vulnerable to.
• Uses existing outage routines.

Cons

• Depends on consistent discipline.
• Does not protect against condensate formed in operation, and puts no barrier on the steel.

Restoring Welds, Heat Tint and Crevices on Existing 316 Stainless Steel

Weld and crevice repair fixes the details where attack starts first.

Suited To

316 equipment where attack concentrates beside welds, at lap joints, fasteners, gaskets or other shielded details.

What It Is

Remove weld heat tint and contamination by pickling and passivation, since heat tint left unpickled lowers local corrosion resistance. Eliminate or seal crevices at overlaps and fasteners where the design allows, and repair cracked welds under a qualified procedure after dye penetrant testing.

Polylloy's technical paper notes that welds, overlaps and fasteners can create shielded locations. 316L reduces weld sensitization but does not improve resistance to chloride pitting.

Pros and Cons

Pros

• Fixes the weakest points directly, and crack repair is needed before any coating anyway.

Cons

• Does not stop general acid attack below the dew point.
• Weld residual stress can still drive chloride stress corrosion cracking; skilled stainless welders and access are required.

Replacing Failed 316 Sections with a Higher Alloy or Coated Carbon Steel

Replacement is for metal that protection can no longer save.

Suited To

Sections perforated, cracked beyond repair or thinned past their allowance.

What It Is

Cut out failed sections and replace them with a higher alloy, such as duplex, super-austenitic or nickel-chromium-molybdenum alloys selected for the actual condensate, or with coated carbon steel. Coated carbon steel avoids the passive-film mechanism where the grade suits temperature and chemistry, while replacing like with like returns the same metal to the same chemistry.

Protected carbon steel may offer a cost-effective alternative to stainless steel where the coating is suitable for the specified service conditions.

Pros and Cons

Pros

• Restores structural integrity.
• Lets the material match the duty that 316 could not.

Cons

• Longest outage and fabrication lead time; mixed-metal joints need design attention.
• Higher alloys still need selection, and coated carbon steel needs blast access and repair of any damage.

How to Choose a Way to Protect Existing 316 Stainless Steel Equipment

The choice starts with where and how the 316 is being attacked.

Inspect Before You Choose

Record wall loss by ultrasonic thickness testing (UT), pit depth and localized damage alongside average corrosion, since a low average rate can hide a penetration risk. Add dye penetrant checks at welds and a thermography map. Polylloy's industrial inspection and maintenance services cover visual, film thickness and adhesion inspection of coated equipment.

Decision Path

Match the finding to the way:

• No measurable attack, no acidic condensate and low chlorides: leave the 316 alone, because a coating adds little.
• Local attack at cold spots, deposits or welds on sound metal: fix the cause (ways 2 to 4) and coat the affected zones with a grade matched to temperature and chemistry (way 1).
• Widespread pitting on accessible surfaces inside a grade's limit: coat after weld repair and near-white blast.
• Perforation or cracking past repair: replace the section (way 5), then protect or select the new material for the duty.

Existing 316 that shows no attack needs no change. For future builds, compare carbon steel vs 316 stainless steel on a like-for-like basis.

Cost and Next Step

Pricing depends on the operating environment, selected coating system, equipment condition, project size, and application requirements. Lifecycle evaluation considers preparation, application, maintenance, downtime, and replacement alongside the initial material cost. To scope existing stainless equipment, request a coating evaluation with equipment type and substrate, coated area, operating conditions (temperature, gas chemistry, abrasion), current damage, location and outage timing.

FAQs

Can We Coat Equipment We Already Built in Stainless Steel?

Often, yes, where the surface can be abrasive blasted to the grade's preparation standard and the grade suits the temperature and chemistry. Stainless needs the same near-white preparation and surface profile as carbon steel, and condition, access and the outage window decide what is practical. Polylloy's technical team confirms the grade and preparation for each component.

What Surface Preparation Does 316 Stainless Steel Need Before Coating?

The same as carbon steel for Polylloy grades: an SSPC-SP10 near-white abrasive blast with a surface profile above 3 mils (75 µm). Existing stainless also needs deposits and contamination removed and cracked welds repaired before blasting. Smooth or inaccessible surfaces that cannot reach that profile are not candidates for coating.

Why Does 316 Stainless Steel Corrode in Acidic Chloride Condensate?

Chloride ions break down the chromium oxide passive film locally, and acidic condensate stops it from healing. Attack then concentrates as pits, in crevices under deposits, gaskets and lap joints, and, where tensile stress and elevated temperature are present near welds, as chloride stress corrosion cracking. Molybdenum improves resistance but gives 316 no immunity.

Is 316 Stainless Steel Necessary for a Flue Gas Application?

Not automatically: where there is no acidic condensate and chloride levels are low, 316 or even carbon steel may be adequate. Where acidic chloride condensate forms, 316 is at risk, so existing 316 may need protection and future builds should compare a higher alloy and protected carbon steel against 316 on a like-for-like basis.

Which FlueGard Grade Suits Existing Stainless Ducts and Baghouses?

Grade follows temperature: FlueGard-225SQC is rated to 225 °C and listed for baghouses, ESPs, ducts and FGD scrubbers. StackGard-255SQW covers chimneys, ducts and fans to 255 °C, while FlueGard-425S and FlueGard-455CHT cover hotter ducts to 425 °C and 455 °C (850 °F). Polylloy confirms the grade for each component.

Should Failed 316 Sections Be Replaced with the Same Alloy?

Usually not without a diagnosis, because replacing like with like returns the same metal to the same condensate. Depending on temperature, chemistry and access, a higher alloy selected for the actual condensate, or coated carbon steel, may suit the duty better. Mixed-metal joints and the outage length belong in the decision.

How Do We Know If Existing 316 Equipment Needs Protection?

Inspect for localized damage, not just average loss. Ultrasonic thickness readings, pit depth measurements, dye penetrant checks at welds and a thermography map of cold spots show where attack is active. A low average corrosion rate can hide deep pits, so record localized damage separately before choosing a fix.