Cyclic corrosion testing

Unlike traditional salt spray tests that maintain a steady-state environment, cyclic corrosion tests cycle between different conditions. When exposing test samples to varying conditions, cyclic corrosion testing better represents the complex corrosive processes materials face in field of use.

These conditions often include:

• Salt spray exposure (exposure to salt solution)
• Humidity phases (high moisture levels)
• Drying phases (low humidity)
• Temperature fluctuation (typically ranging from 25-60°C as seen in automotive standards. In other segments like aviation industry, you may find temperatures reaching from -60 to +180°C)

Each condition has specific parameters like temperature, humidity level, and duration of exposure, which are defined in test standards. Controlled temperature is crucial in cyclic corrosion testing to accurately simulate real-world conditions and assess the long-term durability of materials under different climates.

A common cycle might involve a period of salt spray followed by a period of drying, then a period of humidity, and so on. The sub-cycles typically span form hours to a day, then repeated a few times for one week. The weekly cycles are repeated to obtain a certain level of influence on the material.

The cycling of these conditions aims to better mimic the real-world exposure that materials might experience, such as exposure to salt spray followed by periods of drying and humidity. This dynamic simulation provides a more accurate assessment of how materials will perform in real-world scenarios.

Cyclic corrosion tests provide a more realistic simulation of natural corrosion than traditional static tests. Mimicking natural environmental conditions more accurately leads to more realistic assessments of corrosion resistance, corrosion mechanisms and material degradation.

The relative corrosion rates of structures, and morphologies observed in accelerated cyclic tests tend to mirror those found in real-world scenarios more accurately than steady-state methods.

By accelerating the corrosion process, these tests can provide faster results compared to long-term outdoor exposure studies. The primary goal of cyclic corrosion testing is to achieve accelerated testing results that more closely correlate with outdoor exposure outcomes.

Correlation is a key factor, and the time factor is the second parameter where acceleration grants laboratory testing in favor of onsite exposure which is ineffective and time-wasting.

By identifying potential corrosion issues early on, manufacturers can make design or material changes that lead to reduced maintenance costs for consumers.

This improved correlation makes cyclic testing particularly valuable for industries where long-term material performance is critical, such as automotive, aerospace, military, offshore, paint, plastics, packaging, electronics, and marine applications.

Many industries have developed their own cyclic corrosion test standards to meet specific corrosion resistance needs and material requirements:

• Automotive: VW PV1210, Jaguar Land Rover TPJLR.52.265, Ford Cyclic CETP 00.00 L-467
• Commercial Vehicles: Volvo Trucks ACT 423-0014, Volvo Cars ACT 2 1027,1449, Scania ACT 4319/ACT2 STD4445
• General Standards: ASTM G85, ASTM G44, ASTM D5894

Mercedes, Toyota and many other OEMs have the same approach when it comes to testing though the actual climate cycle can vary between the OEMs as well as the assessments.

It all started at the beginning of the 20th century with an ancient test method where salt fog was sprayed onto test specimens under constant conditions and temperature. ASTM B-117 was developed and for a long time remained unchanged in its design. The salt content in the applied electrolyte started at 25% NaCl, but in following decades, the method was changed by lowering the salt content to today’s level at 5% NaCl.

The traditional salt spray test was revolutionary as you could “get answers” to what was a fully protected metal and what lacked full protection. However, the constant state methods are vague or even faulty in response to the proposed behavior, and what the corrosion mechanisms materials typically experience during their life cycle.

Cyclic corrosion testing represents a significant advancement over traditional salt spray testing. While conventional salt spray tests maintain a steady-state environment, CCT simulates real-world conditions by varying temperature and humidity levels. This dynamic approach allows CCT to more accurately predict real-world corrosion failures, providing a comprehensive evaluation of various corrosion mechanisms, including general corrosion, galvanic corrosion, and crevice corrosion.

Pattern accelerated laboratory tests provide realistic simulations by mimicking wet and dry cycles, thereby offering better correlations to outdoor corrosion rates and mechanisms compared to traditional salt spray tests.

Cyclic corrosion testing involves a series of procedures designed to replicate real-life environmental conditions within a controlled laboratory setting. Test subjects are exposed to various conditions, including alternating wet and dry phases, salt spray, and fluctuating temperatures, within an enclosed chamber.

This chamber can cycle through different environmental states, accurately simulating the conditions that materials and coatings encounter outdoors. CCT machines are capable of testing for a range of corrosion mechanisms, such as general corrosion, galvanic corrosion, and crevice corrosion.

Cyclic corrosion testing is suitable for comparative testing in the optimization of surface treatment systems, articles and components. It is a universal methodology when it comes to testing complex products and geometries. Another key aspect of cyclic corrosion testing lies in its ability to evaluate various corrosion mechanisms simultaneously, providing manufacturers and researchers with a more universal understanding of complex components, protective coatings, and material’s corrosion behavior in significant surroundings.

By replicating the complex interplay of environmental factors, these tests can assess general corrosion, galvanic corrosion, edge corrosion, propagation from scribe, and crevice corrosion within a single test cycle. This broad spectrum of evaluation makes testing time-effective and provides comprehensive insights.

Today, the vast majority of researchers agree that synthetic weathering-like cyclic corrosion test methods should dominate results-based quality research regarding corrosion and corrosion-protective coatings. When verifying the quality of any coating, it is the intended environment in the field of use that makes the method reliable.