Core Purpose
Evaluate corrosion resistance: Verify the corrosion resistance of materials (e.g., metals) and surface treatment layers (e.g., platings, coatings) in salt spray environments, and determine whether they can withstand chloride ion attack (chloride ions are the primary factor causing electrochemical corrosion of metals).
Compare material/process performance: Under identical test conditions, compare the anti-corrosion effectiveness of different platings (e.g., zinc plating, chromium plating), coatings (e.g., anti-rust paint, marine paint), or processes (e.g., electroplating, phosphating) to guide material selection.
Expose potential defects: Detect surface treatment uniformity and adhesion (e.g., pinholes in plating, blistering of coatings), or structural design flaws (e.g., whether crevices are prone to salt spray accumulation).
Meet industry standards: Most products (e.g., automotive components, marine equipment) must pass specific salt spray test standards before market entry (e.g., products sold in coastal areas require higher salt spray resistance requirements).
Common Test Types and Applicable Scenarios
1. Neutral Salt Spray Test (NSS)
Solution composition: 5% sodium chloride (NaCl) aqueous solution (close to seawater salinity), pH adjusted to 6.5~7.2 (neutral).
Test conditions: Salt spray chamber temperature maintained at 35 degree C, with salt mist continuously sprayed onto the specimen surface.
Characteristics: The most fundamental and widely used salt spray test, simulating natural coastal or mild industrial salt spray environments.
Applicable scenarios: General metal products (e.g., hardware components, steel structures), platings (e.g., zinc plating), and preliminary corrosion resistance evaluation of general-purpose coatings.
2. Acetic Acid Salt Spray Test (ASS)
Solution composition: 5% sodium chloride solution with added acetic acid, pH adjusted to 3.1~3.3 (acidic).
Test conditions: Temperature 35 degree C, with continuous salt spray application.
Characteristics: The acidic environment accelerates corrosion, making it more severe than NSS, simulating industrial zones (with acidic pollutants) or coastal environments with high humidity and high salt spray.
Applicable scenarios: Decorative platings of copper, nickel, chromium, etc. (e.g., furniture plating, automotive decorative parts), or outdoor metal components with higher requirements.
3. Copper-Accelerated Acetic Acid Salt Spray Test (CASS)
Solution composition: 5% sodium chloride solution with added acetic acid and copper chloride (CuCl2-2H2O), pH 3.1~3.3, copper chloride concentration 0.26 g/L (accelerated corrosion).
Test conditions: Temperature elevated to 50 degree C (high temperature + acidic + copper ion catalysis, corrosion rate significantly faster than NSS and ASS).
Characteristics: The highest corrosion intensity, short test duration (typically several hours to a few days), suitable for rapid evaluation of platings with high corrosion resistance requirements.
Applicable scenarios: Decorative nickel-chromium platings (e.g., bathroom hardware, automotive chrome-plated parts), high-corrosion-resistance platings (e.g., multi-layer nickel platings).
4. Cyclic Salt Spray Test
Definition: Combines alternating cycles of salt spray, high temperature/high humidity, and drying (e.g., "salt spray 8 h -> high temperature/high humidity 4 h -> drying 4 h" as one cycle), more closely replicating the dynamic "salt spray deposition - wetting - drying" corrosion process found in natural environments.
Characteristics: More representative of real-world service conditions than static salt spray (e.g., outdoor equipment experiencing diurnal temperature variation, wet-dry cycling), capable of exposing corrosion defects caused by temperature/humidity changes (e.g., salt spray penetration after coating cracking).
Applicable scenarios: Automotive components (e.g., chassis parts, fasteners), marine equipment, outdoor communication base stations, and other products exposed to complex environments over long periods.
Key Test Parameters
Salt solution concentration: Typically 5% (+-1%) sodium chloride (simulating seawater); special scenarios may use 3% or 10%.
pH value: NSS (6.5~7.2), ASS/CASS (3.1~3.3), adjusted using hydrochloric acid or sodium hydroxide.
Temperature: NSS/ASS at 35 degree C +-2 degree C, CASS at 50 degree C +-2 degree C; cyclic testing requires additional high-humidity (e.g., 40 degree C, 95% RH) and drying (e.g., 60 degree C) temperature settings.
Salt spray fallout rate: The amount of salt mist deposited on the specimen surface per unit time, typically required to be 1~2 mL/(80 cm2-h) (to ensure stable corrosion intensity).
Test duration: Set according to corrosion resistance requirements, ranging from as short as 24 hours (basic screening) to several thousand hours (e.g., marine paint requiring 1000+ h); CASS is typically 2~96 hours, and cyclic testing runs 10~100 cycles.
Specimen condition: Whether pre-conditioned (e.g., scribed, bent, to simulate damage during transport/service), and placement angle (typically 15 degrees ~30 degrees from vertical, to ensure uniform salt spray coverage).
Applicable Products and Industries
Metal products: Hardware (screws, nuts), steel structures (bridges, guardrails), metal sheets;
Platings / coatings: Electroplated layers (zinc plating, chromium plating, nickel plating), coatings (anti-rust paint, marine paint, automotive paint), anodized layers (aluminum alloys);
Automotive industry: Chassis components (bolts, bearings), body plating, wiring harness connectors (to prevent poor contact);
Marine / shipbuilding: Hulls, anchor chains, offshore platform components (high salt spray environment);
Electronics and electrical: Connectors, circuit boards (to prevent solder joint corrosion leading to short circuits), outdoor lighting fixtures;
Construction industry: Door and window hardware, curtain wall metal components (coastal buildings require high salt spray resistance).
Key Test Standards
General standards:
ISO 9227 (international standard, covering NSS/ASS/CASS);
GB/T 10125 (Chinese national standard, equivalent to ISO 9227);
ASTM B117 (US standard, neutral salt spray test).
Industry standards:
Automotive: ISO 16750 (road vehicles - electrical and electronic equipment), SAE J2334 (automotive coating salt spray test);
Electronics: IEC 60068-2-11 (environmental testing: salt mist);
Platings: ISO 1456 (metallic coatings salt spray test), GB/T 2423.17 (salt spray testing for electric and electronic products).
Test Result Evaluation Criteria
Degree of corrosion:
Rusted area: e.g., "rusted area less than or equal to 5%" is considered qualified (to be graded per standards such as ISO 10289 rust rating);
Plating/coating condition: whether blistering, peeling, cracking, or under-plating corrosion has occurred (e.g., white rust or red rust appearing on zinc-plated layers).
Substrate protection capability: After plating/coating failure, whether the substrate (e.g., steel) rapidly corrodes (time to red rust appearance is a key indicator, e.g., zinc-plated parts require no white rust within 48 h and no red rust within 96 h).
Functional impact: Whether electronic components experience short circuits or poor contact due to corrosion; whether mechanical parts seize due to rust.