Corrosion Resistance and Surface Finishing of Aluminum 7175 Sheet

In aerospace, defense, and high-performance engineering industries, material selection plays a crucial role in achieving strength, durability, and reliability. Aluminum 7175 Sheet has emerged as one of the most trusted materials thanks to its exceptional strength-to-weight ratio, superior fatigue resistance, and good corrosion resistance.

However, to ensure long-term service performance—especially in harsh or marine environments—understanding the alloy’s corrosion behavior and surface finishing treatments is vital. Proper finishing not only enhances corrosion protection but also improves aesthetics, adhesion, and fatigue life.

In this article, SASAALUMINUM examines the corrosion resistance and surface finishing techniques of Aluminum 7175 Sheet, exploring how advanced treatments enhance durability, functionality, and visual quality for aerospace and industrial applications.

Overview of Aluminum Alloy 7175

Aluminum Alloy 7175 belongs to the high-strength 7xxx series (Al-Zn-Mg-Cu system). It was developed as an improved version of 7075, offering better fracture toughness, stress-corrosion resistance, and machinability.

Typical Chemical Composition (%):

• Zinc (Zn): 5.6–6.1

• Magnesium (Mg): 2.0–2.5

• Copper (Cu): 1.2–1.6

• Chromium (Cr): 0.18–0.25

• Aluminum (Al): Balance

Through precipitation hardening, this alloy achieves very high strength comparable to steel while remaining lightweight. It is commonly used in aircraft wing spars, fuselage frames, landing gear components, and other load-bearing structures.

At SASAALUMINUM, Aluminum 7175 Sheets are manufactured under strict quality control, with precision heat treatment and surface finishing to meet the rigorous demands of aerospace standards.

Corrosion Resistance of Aluminum 7175 Sheet

Although Aluminum 7175 offers good natural corrosion resistance due to the formation of a passive aluminum oxide (Al₂O₃) film on its surface, its high zinc and copper content make it more prone to localized corrosion (such as pitting or stress corrosion cracking) than lower-strength alloys.

Thus, understanding its corrosion behavior and applying suitable surface protection methods are key to ensuring durability.

Types of Corrosion in Aluminum 7175

1. Pitting Corrosion

   • Occurs when the protective oxide layer breaks down in chloride-rich environments (e.g., seawater).

   • Small pits form on the surface, which can propagate with time.

2. Intergranular Corrosion (IGC)

   • Results from segregation of alloying elements (especially zinc and copper) along grain boundaries.

   • More common in alloys that are improperly heat-treated or over-aged.

3. Stress-Corrosion Cracking (SCC)

   • A combination of tensile stress and a corrosive environment can cause cracking over time.

   • T73 or T74 tempers are designed specifically to resist this type of corrosion.

4. Galvanic Corrosion

   • Occurs when Aluminum 7175 is in contact with dissimilar metals (like steel or copper) in a conductive medium.

   • Mitigation requires proper insulation or coating systems.

Factors Affecting Corrosion Resistance

1. Alloy Composition – Higher copper improves strength but slightly decreases corrosion resistance.

2. Heat Treatment Temper – Overaged tempers (T73, T74) improve resistance compared to T6.

3. Surface Condition – Smooth, clean, and properly coated surfaces resist corrosion more effectively.

4. Environmental Exposure – Humidity, salt concentration, and temperature influence corrosion rates.

To maintain high durability, SASAALUMINUM optimizes both heat treatment and surface finishing for every Aluminum 7175 Sheet, balancing strength with long-term corrosion protection.

Improving Corrosion Resistance Through Heat Treatment

The heat treatment condition significantly affects corrosion behavior:

1. T6 Temper (Solution Treated and Aged)

• Provides maximum strength and hardness.

• Slightly lower corrosion resistance due to fine precipitates at grain boundaries.

2. T73 Temper (Over-Aged)

• Heat-treated at higher temperatures for longer durations.

• Coarser precipitates reduce susceptibility to stress-corrosion cracking.

• Preferred for humid and marine environments.

3. T74 Temper (Intermediate Aging)

• Offers balanced strength and corrosion resistance.

• Commonly used in aerospace applications requiring both properties.

SASAALUMINUM provides Aluminum 7175 Sheets in multiple tempers (T6, T73, T74), allowing customers to choose based on strength and environmental requirements.

Surface Finishing of Aluminum 7175 Sheet

To enhance corrosion protection and improve surface performance, various finishing techniques are applied to Aluminum 7175 Sheets. Each technique provides a specific layer of defense and improves the overall functionality and lifespan of the material.

1. Mechanical Finishing

Mechanical surface treatments improve smoothness, remove impurities, and prepare the surface for further coating.

Common Methods:

• Grinding and Polishing: Achieves mirror-like finish and removes surface oxides.

• Brushing: Creates a uniform satin texture while hiding surface defects.

• Shot Peening: Introduces compressive stress to improve fatigue resistance.

Mechanical finishing also enhances the adhesion of subsequent coatings or anodized layers.

2. Chemical Cleaning and Etching

Before anodizing or coating, Aluminum 7175 sheets undergo chemical treatment to remove oxides, oils, and contaminants.

Processes Include:

• Alkaline Cleaning: Removes organic residues and oils.

• Acid Etching: Provides uniform surface roughness for better coating adhesion.

• Desmutting: Eliminates residual smut particles after etching using nitric acid-based solutions.

These steps ensure a chemically clean surface ready for high-quality anodizing or conversion coating.

3. Anodizing – The Most Effective Corrosion Protection

Anodizing is an electrochemical process that thickens the natural oxide layer on the aluminum surface, offering excellent corrosion resistance and wear protection.

Types of Anodizing Used for 7175 Aluminum:

• Type II (Decorative Anodizing):

  • Produces an oxide layer 5–25 µm thick.

  • Can be dyed for aesthetic purposes.

  • Suitable for interior or low-corrosion environments.

• Type III (Hard Anodizing):

  • Creates a dense oxide film up to 50–100 µm thick.

  • Provides outstanding wear and corrosion resistance.

  • Commonly used in aerospace and defense components.

Advantages of Anodizing:

• Superior corrosion resistance, especially in marine environments.

• Enhanced hardness and wear resistance.

• Excellent base for paint or adhesive bonding.

• Environmentally stable and long-lasting surface finish.

At SASAALUMINUM, all anodizing processes conform to MIL-A-8625 and ASTM B580 aerospace standards, ensuring consistent thickness and color uniformity.

4. Chemical Conversion Coating (Alodine/Chromate Treatment)

Also known as chromate conversion coating, this process offers a conductive yet corrosion-resistant surface.

Process Details:

• Immersion in chromate solution forms a thin film (0.3–0.8 µm) on the aluminum surface.

• The coating provides sacrificial corrosion protection and excellent paint adhesion.

• Commonly used when electrical conductivity and bonding compatibility are required.

SASAALUMINUM provides RoHS-compliant trivalent chromate coatings for environmentally friendly applications without compromising performance.

5. Painting and Powder Coating

After anodizing or conversion coating, painting or powder coating can be applied for additional protection and visual appeal.

Benefits Include:

• Extended corrosion protection for outdoor and marine environments.

• Customizable colors and finishes.

• Added layer of mechanical protection against scratches and abrasion.

These coatings are cured and inspected for adhesion, hardness, and thickness consistency to ensure aerospace-grade quality.

Surface Finishing Comparison