مبدأ تخليل وتخميل الفولاذ المقاوم للصدأ

1. Corrosion Resistance of Stainless Steel

The corrosion resistance of stainless steel is primarily due to its high chromium (كر) content. على سبيل المثال, في الفولاذ المقاوم للصدأ 316L, chromium content ranges between 16.00% و 18.00%. Chromium easily oxidizes to form a thin, dense Cr₂O₃ oxide film on the surface. This film significantly increases the steel’s electrode potential, enhancing its resistance to corrosion in oxidizing environments.

The protective film, تقريبًا 1 nm thick, serves as a barrier against corrosive substances. لكن, if the film is incomplete, damaged, or defective, the underlying stainless steel becomes susceptible to corrosion.

2. Challenges to Stainless Steel Surfaces

During the fabrication and processing of stainless steel chemical tankers, the following contaminants and damages can occur:

Surface impurities: زيت, rust, and low-melting-point metal pollutants.
Mechanical damage: Scratches, ضربة اللحام, and splashes.
Inspection residues: Paint and substances from flaw detection or pressure tests.

These factors compromise the passivation film, reducing corrosion resistance and exposing the material to risks like pitting, التآكل الحبيبي, and stress corrosion cracking.

Seawater tank cleaning during operation adds to the challenge. Chloride ions in seawater accelerate corrosion by attacking the passivation film. Under such harsh conditions, pickling and passivation become essential maintenance processes.

3. Dynamic Nature of the Passivation Film

The passivation film on stainless steel is not static. It acts as a dynamic protective layer that responds to the surrounding environment:

Damage: Reducing agents, such as chloride ions, can degrade the film.
Repair: Oxidizing agents, like air, help restore the film by forming new protective layers.

Although stainless steel naturally forms an oxide film in air, this layer often lacks sufficient protective strength.

4. Effectiveness of Pickling and Passivation

Pickling removes approximately 10 μm of the stainless steel’s surface. The chemical activity of the acid ensures defective areas dissolve faster, resulting in a smoother, more uniform surface. This process also achieves the following:

Selective dissolution: Iron and iron oxides are removed preferentially over chromium and chromium oxides.
Chromium enrichment: The surface becomes chromium-rich by eliminating chromium-deficient layers.

تبعًا, an oxidizing agent is applied, forming a complete and stable passivation film. The potential of this chromium-rich layer can reach +1.0V (SCE), comparable to precious metals, enhancing its corrosion resistance and stability.

5. خاتمة

يعد التخليل والتخميل أمرًا ضروريًا للحفاظ على مقاومة الفولاذ المقاوم للصدأ للتآكل, خاصة في البيئات الصعبة. تعمل هذه العمليات على استعادة وتعزيز طبقة التخميل الواقية, ضمان متانة ووظيفة مكونات الفولاذ المقاوم للصدأ في ناقلات المواد الكيميائية والتطبيقات الأخرى.