PP Packing Survives but Stainless-Steel Supports Corrode in a Chloride-Rich Scrubber

A scrubber can have excellent packing life and still suffer a serious internal failure.

This occurs when the packing and structural internals are made from different materials.

A common arrangement is:

• PP or other plastic packing;
• metallic support structure;
• metallic fasteners.

The polymer may remain chemically intact while the metal components experience increasingly aggressive chloride-rich liquid.

Project Situation

Consider a packed wet scrubber handling a chloride-containing process.

The tower uses:

• PP random packing;
• stainless-steel support grid;
• stainless fasteners;
• recirculating aqueous liquid.

After several years, inspection shows:

• plastic packing still looks usable;
• metal support members show localized corrosion;
• bolts or clips are heavily attacked.

The client asks:

“If PP survived, why did the internal system fail?”

Because the packing material and structural material were facing different corrosion mechanisms.

Recirculation Can Concentrate Chlorides

The fresh liquid entering the system may contain only moderate chloride.

During operation:

• acidic gas is absorbed;
• water evaporates;
• salts remain.

If blowdown is limited, chloride concentration in the circulation loop can increase substantially.

The final service can therefore be much more aggressive than the startup-water analysis suggests.

Stainless Steel Is Not a Universal Chloride Material

Stainless steels rely on a passive surface film.

Chloride-containing environments can create localized corrosion risks depending on:

• chloride concentration;
• temperature;
• pH;
• alloy grade;
• oxygen condition;
• deposits.

The word “stainless” is therefore not sufficient as a corrosion specification.

Crevices Are Especially Important

Fasteners and support assemblies contain:

• bolt interfaces;
• overlaps;
• gasketed joints;
• deposit-covered surfaces.

These areas can experience chemistry different from the bulk liquid.

A flat exposed coupon can look acceptable while creviced hardware deteriorates.

Temperature Increases the Risk

A tower that operated reliably at moderate temperature may develop accelerated metal attack after:

• hotter gas;
• warmer recirculation;
• process expansion.

The PP packing may still remain within its chemical and thermal range.

The metal-support margin can change independently.

Deposits Can Create Local Corrosion Cells

Salt, sludge, or biological deposit on a support member can create a shielded area beneath the deposit.

Local chemistry may become more aggressive than the well-mixed sump.

Therefore, fouling is not only a hydraulic concern.

It can alter corrosion behavior.

Support Failure Is More Serious Than Packing Surface Damage

A degraded packing element may reduce efficiency locally.

A weakened support can threaten:

• the whole bed;
• the distributor below;
• safe operation.

Structural internals should therefore receive their own corrosion review rather than inheriting the packing material decision.

Fasteners Need the Same Attention

Projects sometimes specify a corrosion-resistant main grid but use ordinary or lower-grade:

• bolts;
• washers;
• clips.

These small items can become the earliest failure points.

The material schedule should identify every wetted component.

“All Plastic” Can Be a Design Direction—but Has Its Own Limits

Replacing metal with PP, FRP, PVDF, or another nonmetallic system can reduce chloride-corrosion risk in suitable service.

However, nonmetallic supports have different:

• strength;
• creep behavior;
• temperature limits;
• span requirements.

The solution should be engineered, not treated as a simple one-for-one material swap.

Inspection Should Include Thickness and Connection Condition

A visual check of the packing alone is not enough.

During shutdown inspect:

• beams;
• support grid;
• perimeter connections;
• clips;
• bolts.

Look especially at:

• liquid-retaining areas;
• deposits;
• crevices.

Process Liquid Should Be Tested at Mature Operation

Material selection should use the real circulating-liquid chemistry after the system reaches its normal concentration.

Useful values include:

• chloride;
• pH;
• temperature;
• dissolved solids;
• oxidizing species where relevant.

Fresh makeup water is not the same design basis.

Engineering Takeaway

The most corrosion-resistant visible component is not necessarily the component that determines tower life.