Filiform corrosion testing evaluates the resistance of organic-coated metals—primarily aluminum alloys and coated steel—to thread-like underfilm corrosion that develops in humid, chloride-bearing environments. The test is performed in accordance with internationally recognized standards such as ASTM D2803, ISO 4623-2, and EN 3665, and is widely used for qualification of aerospace, automotive, and architectural coating systems. This article explains the testing principle, sample requirements, test procedures, evaluation criteria, and typical industrial applications.
Technical Overview
Filiform corrosion (also known as filamentary corrosion or underfilm corrosion) is a localized form of corrosion that occurs beneath organic coatings on metal surfaces. It appears as fine, thread-like filaments that propagate from coating defects, cut edges, or scratches, typically measuring less than 1 mm in width but extending several millimeters in length. Although filiform corrosion rarely causes perforation, it degrades appearance, undermines coating adhesion, and can act as an initiation site for more severe forms of corrosion such as pitting or exfoliation.
The mechanism of filiform corrosion is driven by differential aeration cells. The head of each filament is anodic, oxygen-depleted, and acidic due to metal ion hydrolysis, while the trailing tail is cathodic and alkaline. Propagation requires a narrow humidity window—generally between 60% and 95% relative humidity—combined with chloride contamination at the initiation point, which is why the test combines a chloride initiation step with controlled humidity exposure.
Filiform corrosion testing is a laboratory accelerated test designed to reproduce this damage mechanism and quantify the susceptibility of coating systems on aluminum alloys, steel, and galvanized steel. The principal standards include:
- ASTM D2803 – Standard Guide for Testing Filiform Corrosion Resistance of Organic Coatings on Metal
- ISO 4623-1 – Filiform corrosion test on steel substrates
- ISO 4623-2 – Filiform corrosion test on aluminum alloys
- EN 3665 – Aerospace series test method for filiform corrosion on aluminum alloys
- Qualicoat and other industry specifications referencing the EN 3665-type procedure
Testing Samples
Organic-Coated Aluminum Alloys
The most common test articles are painted, lacquered, or anodized-plus-coated aluminum alloys, including aerospace skin alloys (2xxx and 7xxx series), automotive body sheet (5xxx and 6xxx series), and extruded architectural profiles (6060/6063). The conversion coating or pretreatment (chromate, chromium-phosphate, or chromium-free pretreatments) plays a decisive role in test outcome, so samples must represent the full production coating system.
Coated Steel and Galvanized Steel
Coil-coated steel sheet, hot-dip galvanized or galvannealed steel with organic finishes, and painted fabricated steel parts are tested to ISO 4623-1 or ASTM D2803. Typical products include building panels, appliance housings, and fastener coatings.
Sample Preparation Requirements
Test panels are typically flat coupons (for example, approximately 100 mm × 150 mm or 75 mm × 150 mm) with the complete coating system applied under representative production or laboratory conditions. Before testing, a scribe line or cross is cut through the coating to the bare metal substrate using a standardized scribing tool. Scribing provides a deliberate coating defect through which chloride ions can reach the metal surface and initiate filaments. Edges and the reverse side are usually masked to restrict corrosion to the scribed test area, and replicate panels are tested to ensure statistical reliability.
Testing Items
The test program assesses the following characteristics of the coating–substrate system:
- Filament initiation: whether filaments start from the scribe line after the chloride initiation step
- Filament length: the average and maximum length of filaments measured from the scribe, typically in millimeters
- Filament density: the number of filaments per unit length of scribe or per unit area
- Filament width and morphology: thread diameter, branching, and propagation direction
- Underfilm degradation: blistering, coating adhesion loss, or delamination along the filament tracks
- Rating classification: assignment of a numerical rating or pass/fail judgment against the applicable specification limit
Testing Methods
ASTM D2803 Procedure
Scribed panels are first exposed to a chloride-containing medium—commonly immersion in sodium chloride solution or a brief salt-fog exposure—to deposit chloride ions at the bare metal. The panels are then transferred to a controlled-humidity cabinet, typically maintained at approximately 85% relative humidity at the temperature specified in the method, and held for an agreed period, commonly up to 1000 hours (about six weeks). Filaments that develop are measured and recorded at defined intervals.
ISO 4623 Procedures
ISO 4623-1 for steel substrates and ISO 4623-2 for aluminum alloys follow a similar two-stage approach. For aluminum, scribed specimens are commonly exposed to hydrochloric acid vapor or an acidified chloride solution to initiate attack, followed by storage in a humidity chamber at approximately 40 °C and 80–85% relative humidity for the specified test duration. The acidic chloride initiation closely reproduces the conditions that trigger filiform attack on aluminum in service.
EN 3665 Aerospace Method
EN 3665 is the reference method for aerospace coating systems on aluminum alloys. Scribed specimens are exposed for about one hour to the vapor of concentrated hydrochloric acid to initiate corrosion at the scribe, then placed in a climatic chamber at approximately 40 °C with relative humidity in the range of roughly 80–85% for a standard duration of 1000 hours. The test is a mandatory qualification element for many aircraft original equipment manufacturer specifications and for paint system approvals under quality labels such as Qualicoat.
Evaluation and Reporting
After exposure, panels are inspected under adequate illumination, often with low-power magnification. The lengths of all visible filaments growing from the scribe are measured, and the average and maximum filament lengths are reported. Results are expressed either as direct measurements or converted into rating numbers. Acceptance limits vary by specification; typical requirements specify maximum filament lengths in the range of about 0.5 mm to 2 mm after 1000 hours of exposure, with longer filaments indicating weaker pretreatment or coating protection.
Applications
Filiform corrosion testing supports material selection, process control, and regulatory qualification across multiple industries:
- Aerospace: qualification and batch control of primer and topcoat systems on aircraft aluminum structures per EN 3665 and OEM specifications
- Automotive: validation of pretreatment, e-coat, and paint systems on aluminum body sheet and closures exposed to de-icing salts and road humidity
- Architecture and construction: approval of powder-coated and liquid-coated aluminum profiles and coil-coated façade panels under Qualicoat and similar quality labels
- Packaging: assessment of lacquered aluminum and steel can stock resistance to underfilm attack in humid transport and storage conditions
- Coatings development: comparative evaluation of pretreatment chemistries, primer formulations, dry-film thicknesses, and cure conditions
- Failure analysis: reproducing service filiform attack in the laboratory to identify root causes such as inadequate pretreatment or insufficient coating thickness
FAQ
Question: What is the difference between filiform corrosion testing and salt spray testing? Answer: Salt spray testing (for example, ASTM B117) exposes samples continuously to a chloride fog and drives general corrosion mechanisms, whereas filiform corrosion testing combines a brief chloride initiation step with long-term exposure to controlled humidity without liquid condensation, specifically promoting the differential-aeration mechanism of filamentary underfilm attack.
Question: How long does a filiform corrosion test take? Answer: The most common test duration is 1000 hours (approximately six weeks) of humidity exposure after initiation, although shorter screening periods of 24 to 500 hours are also used depending on the specification.
Question: Which substrates are most susceptible to filiform corrosion? Answer> Coated aluminum alloys, magnesium alloys, and coated or galvanized steels with organic coatings are the most susceptible, particularly when pretreatment quality is poor or the coating is thin or damaged.
Question: What factors influence the test result? Answer> Key factors include the pretreatment or conversion coating type, coating thickness and adhesion, scribing quality, cleanliness of the substrate, the humidity and temperature control of the chamber, and the severity of the chloride or acid initiation step.
Question: How are results judged as pass or fail? Answer: The average and maximum filament lengths measured from the scribe are compared with the limit stated in the applicable product specification, such as a maximum filament length of 2 mm after 1000 hours for certain aerospace and architectural qualification schemes.