Expert Stainless Steel Solutions

Preserving Integrity Below Deck

Stainless Steel Corrosion: Identification & Solutions

In the harsh marine environment, even high-grade Stainless Steel (316L/316LN) and Duplex are susceptible to specific types of degradation. Understanding the root cause is essential for a permanent repair. TECHYARA Marine specializes in diagnosing these issues and providing long-term restoration solutions.

Common Corrosion Types in Marine Tanks & Systems

1. Pitting Corrosion

The Problem: Small, deep holes that penetrate the protective passive layer, often caused by chlorides (seawater) or stagnant acidic cargo residues. It is "cancerous" to the steel; if not treated, it can lead to full plate penetration.
The TECHYARA Solution: * Mechanical excavation of the pit using specialized Power Buffing.
1. In-depth cleaning to remove chlorides from the "pit floor."
2. Verification via Dye-Check to ensure the defect is fully removed before chemical passivation.

2. Crevice Corrosion

The Problem: Occurs in shielded areas where oxygen cannot reach the metal surface, such as under gaskets, bolt heads, or scale deposits (e.g., in heating coils or flange connections).
The TECHYARA Solution: * Disassembly and thorough mechanical cleaning of the affected areas.
1. Polishing the crevice surfaces to a high-gloss (low Ra) finish to minimize future entrapment.
2. Replacement of contaminated gaskets and application of specialized inhibitors.

3. Intergranular Corrosion (Sensitization)

The Problem: Often found near weld seams (the Heat Affected Zone). If welding wasn't performed correctly, chromium carbides precipitate, leaving the steel vulnerable to rapid decay.
The TECHYARA Solution: * Professional grinding of the affected weld zones.
1. Specialized chemical pickling to remove the chromium-depleted layer.
2. Expert supervision of any necessary re-welding to ensure correct heat input and gas shielding.

4. Stress Corrosion Cracking (SCC)

The Problem: A combination of tensile stress and a corrosive environment (often involving chlorides and high temperatures, typical for Marine Scrubbers and Heating Coils).
The TECHYARA Solution: * Detailed NDT (Non-Destructive Testing) to map the extent of the cracks.
1. Consultancy on stress-relief measures.
2. Full-depth repair or section replacement under strict quality control.

5. Ferrous Contamination ("Rust Smut")

The Problem: Not "true" corrosion of the stainless steel itself, but iron particles from tools or nearby carbon steel work that "burn" into the stainless surface and start rusting.
The TECHYARA Solution
: * Ferroxyl Testing to identify the contamination.
1. De-rusting using specialized gels.
2. Complete passivation to restore the 100% stainless integrity.

The TECHYARA Restoration Process

To fix these issues permanently, we follow a rigorous 3-step protocol:

Diagnosis: We use Ra-meters, Oxilyzers, and Ferroxyl tests to determine the exact state of the metal.
Mechanical Intervention: Using Power Buffing and precision grinding, we remove the damaged layers until we reach "healthy" parent metal.
Chemical Restoration: We apply the final Pickling and Passivation treatment to recreate the protective chromium-oxide layer that makes stainless steel "stainless."

Technical Support on Demand

Do you suspect corrosion in your cargo tanks or scrubber system? TECHYARA Marine provides Global Inspection Surveys to assess the damage before it leads to cargo contamination or off-hire time.

The WPA Challenge: Dealing with Wet Phosphoric Acid Contamination

"Prevent weeks of failed wall wash tests. TECHYARA Marine specializes in 'First-Time-Right' cleaning and restoration after WPA cargoes." Transporting Wet Phosphoric Acid (WPA) is one of the most demanding operations for stainless steel cargo tanks. Beyond its corrosive nature, WPA is notorious for leaving behind a persistent byproduct that can jeopardize future high-purity cargoes.

The Problem: The "Infinite" Black & Grey Dust

After discharging WPA, crews often face a phenomenon known as "Grey/Black Smut" or "Post-Cargo Dust."

The Layer: A complex mixture of calcium sulfates (gypsum), fluorides, and metallic phosphates forms a hard, porous scale on the tank walls.

The "Infinite" Dust: Even after multiple hot water washes and chemical cleanings, the tank walls can continue to release a fine black or grey powder. This occurs because the acid has microscopically etched the surface, trapping particles in the grain boundaries of the steel.

The Risk: This dust causes "Wall Wash" failures for subsequent cargoes (like Methanol or Glycols) and acts as a catalyst for localized pitting corrosion if left untreated.

The TECHYARA Solution: Mechanical & Chemical Restoration

Standard cleaning methods often fail to reach the "root" of WPA contamination. TECHYARA Marine provides a specialized restoration protocol:

Mechanical De-Scaling (Power Buffing): When the contamination is embedded in the surface profile, we use controlled power buffing. This removes the "infinite dust" by mechanically leveling the etched peaks and valleys of the steel where the particles hide.

Chemical Pickling (Specialized Acids): We apply a specific blend of nitric and hydrofluoric acids to dissolve the deep-seated phosphate scale and "reset" the stainless steel surface.

Ra-Value Restoration: WPA often increases the surface roughness. We polish the bulkheads back to a mirror-like finish (Ra < 1.5 µm) to ensure the next cargo won't stick.

Verification: After perform a wipe-test and a high-purity wall wash analysis to be done in order to guarantee that the "infinite dust" problem is permanently resolved.

Why Supervision is Key

During WPA restoration, shipyards often over-grind or use incorrect chemicals, which can permanently damage the Duplex or 316L steel. TECHYARA Marine supervisors ensure that the removal process is aggressive enough to eliminate the dust, but gentle enough to preserve the integrity of your tank walls.