LPDC casting salt spray corrosion test, neutral salt spray NSS, CASS, intergranular corrosion, anti-corrosion scheme

LPDC casting salt spray corrosion test, neutral salt spray NSS, CASS, intergranular corrosion, anti-corrosion scheme
Aluminum alloy shells often fail due to long-term outdoor, humid, and refrigerant environments, as well as salt spray corrosion; Many factories only focus on the appearance without white rust, ignoring intergranular corrosion. Corrosion extends inward along grain boundaries, causing slow deterioration of mechanical strength and posing a risk of explosion and leakage in pressure bearing components.
1、 Classification and Quantitative Parameters of Salt Spray Testing (A356)
Neutral salt spray NSS: 5% NaCl solution, pH 6.5–7.2， Spray temperature 35 ℃, sedimentation 1 – 2ml/80cm ² · h; Universal outdoor foundation anti-corrosion acceptance.
CASS copper accelerated acetate salt spray: containing copper chloride, pH 3.1–3.3， 50 ℃, faster corrosion rate, suitable for accelerated verification of anodized and sprayed parts.
Intergranular corrosion test: According to ASTM G67, immerse in mercuric nitrate solution, bend after corrosion, and observe grain boundary cracking. When the magnesium content of A356 is high, the risk of intergranular corrosion increases.
A356 bare aluminum NSS: white rust usually appears within 24-72 hours; Chromium free passivation can last for 200-500 hours; hard anodizing can last for over 1000 hours.
Xinfeng Machinery distinguishes based on customer operating conditions: passivation is recommended for indoor water-cooled shells, while hard anodizing is recommended for outdoor chassis components.
2、 Sample pretreatment, sealing, rating, and corrosion source control
Clean and remove oil before testing, and do not touch the surface of the sample directly with your hands. Oil and grease will form local shielding and cause uneven corrosion.
Threaded holes, sealing grooves, and internal flow channels are protected with specialized sealing wax in areas that are not sprayed. The sealing wax boundary is clear and does not contaminate the test surface.
Rating standard: GB/T 6461, comparing corrosion area and pitting depth; Key control points for pressure bearing parts include the depth of pitting corrosion, which penetrates the wall thickness and is directly scrapped.
Defects in casting itself: looseness, subcutaneous pores, and preferential pitting corrosion occur at the location of inclusions, and corrosion extends outward from the inside of the defect.
3、 Differences in Salt Spray Control for Overseas Customers
Most Southeast Asian customers only use 24-hour neutral salt spray, and a small amount of white rust is acceptable, relying on later painting to cover it up.
North American new energy water-cooled shell, NSS 500h without red rust, with a clear upper limit for pitting depth, and a passivation film thickness report attached.
European outdoor hydraulic components are subject to mandatory intergranular corrosion testing except for NSS, and continuous corrosion channels at grain boundaries are not allowed.
Middle East coastal project, sand and salt spray composite testing, while assessing wear and corrosion, requires thick film anodizing.
4、 Corrosion Failure Defect Chain
Subcutaneous looseness and micropores in castings → infiltration of corrosive media into the interior → internal pitting corrosion, with a slight appearance and continuous thinning of wall thickness;
Magnesium element segregation, enrichment of Mg ₂ Si at grain boundaries → intergranular corrosion, delamination along grain boundaries;
Surface release agent residue, oil stains → local discontinuity of passivation film, priority corrosion;
Insufficient thickness of passivation film and inadequate baking → The film layer is porous and resistant to salt spray, resulting in significant shrinkage.
5、 Long tail traffic layout ideas
Low pressure casting salt spray testing, neutral salt spray NSS, intergranular corrosion, aluminum alloy passivation, facing surface treatment and quality engineers.
keywords LPDC aluminum casting NSS salt spray test、intergranular corrosion A356 casting；
Question category: CASS test requirement for aluminum alloy housing;
Comparison category: Passage vs Hard Anodizing Salt Spray Resistance.
Suitable for surface anti-corrosion and reliability themed pages.
6、 Common misconceptions in the industry
Misconception 1: If there is white rust, it is considered unqualified. The drawing only controls red rust, which is a corrosion product of alumina and does not represent penetration corrosion.
Misconception 2: If the salt spray is qualified, there will be no intergranular corrosion. Salt spray cannot see the grain boundaries, and a separate intergranular corrosion test must be conducted.
Misconception 3: The thicker the film layer, the better. Excessive thickness in anodizing can lead to microcracks and crack propagation under stress conditions.
FAQ
Q: Neutral salt spray NaCl concentration?
A：5%。
Q: NSS testing temperature?
A: 35 ℃.
Q: CASS testing temperature?
A: 50 ℃.
Q: What is the duration of neutral salt spray on A356 bare aluminum?
A: White rust appears within 24-72 hours.
Q: Common standards for intergranular corrosion?
A：ASTM G67。
Q: What is the greatest harm of pitting corrosion?
A: Local wall thickness reduction and leakage of pressure bearing components.
Q: Advantages of passivation film?
A: Low cost, no change in size, suitable for sealing surfaces.
Q: Shortboard of anodizing?
A: The film layer has a thickness, and the sealing holes and assembly dimensions need to be reserved with allowance.