Corrosivity testing determines whether a hazardous chemical can irreversibly destroy skin or eye tissue on contact, or corrode steel and aluminum surfaces, forming the technical basis for UN Class 8 transport classification and GHS hazard communication. This article explains internationally recognized test methods—including OECD in vitro skin models and the UN metal corrosion test—together with sample requirements, packing group criteria, and practical regulatory applications for manufacturers, shippers, and accredited third-party laboratories.
Technical Overview
Corrosivity is one of the core physicochemical and health hazard endpoints used to classify dangerous goods and hazardous chemicals. Under the UN Model Regulations and the Globally Harmonized System of Classification and Labelling of Chemicals (GHS), a substance is considered corrosive when, by chemical action, it causes severe damage to skin, mucous membranes, or eye tissue on contact, or when it corrodes steel or aluminum at a rate exceeding 6.25 mm per year at a test temperature of 55 °C. Substances meeting either criterion are assigned to Class 8 (Corrosive Substances) for transport purposes.
Within the GHS framework, corrosivity is expressed through two main hazard classes: skin corrosion (Category 1, subdivided into 1A, 1B, and 1C based on exposure time and onset of irreversible tissue damage) and serious eye damage (Category 1, involving irreversible effects on the cornea, iris, or conjunctiva that are not fully reversible within 21 days). These classifications directly drive the hazard statements, pictograms, and precautionary statements that appear on labels and Safety Data Sheets, such as the skin corrosion/serious eye damage classifications referenced in the GB 30000 series of standards used for chemical classification and labelling.
Modern corrosivity evaluation follows a tiered, weight-of-evidence strategy. Existing human data, animal data, structure–activity relationships, pH measurements, and buffering capacity are assessed first. Where new testing is required, validated in vitro methods are applied before any confirmatory in vivo study, in line with the 3Rs principle (replacement, reduction, and refinement of animal use). This integrated approach is recognized by regulatory authorities worldwide and is routinely implemented by accredited third-party laboratories supporting global chemical registration and dangerous goods compliance.
Testing Samples
Acids and Acidic Solutions
Inorganic acids such as sulfuric acid, nitric acid, hydrochloric acid, hydrofluoric acid, and phosphoric acid, along with organic acids such as acetic acid, formic acid, and methanesulfonic acid, are among the most frequently submitted sample types. Concentration, water content, and the presence of stabilizers or inhibitors significantly influence corrosive behavior, so representative production batches should be sampled for evaluation.
Alkalis and Basic Solutions
Strong bases including sodium hydroxide, potassium hydroxide, sodium methoxide, alkanolamines, and ammonia solutions are tested to confirm skin and eye corrosion potential and to establish packing groups for transport. Buffering capacity is particularly relevant for alkaline formulations, as substances with pH ≥ 11.5 and substantial alkali reserve may be classified as corrosive without further biological testing.
Oxidizing and Multifunctional Corrosives
Certain chemicals exhibit corrosivity alongside additional hazards, including hydrogen peroxide solutions, hypochlorites, bromine, chromic acid mixtures, and permanganates. These samples often require combined evaluation against multiple hazard classes so that the most restrictive classification and packaging requirements are correctly applied.
Corrosive Solids
Corrosive solids such as sodium hydroxide flakes or pellets, potassium hydroxide, zinc chloride, phenol, and various hygroscopic salts are tested in both dry and moistened forms. Because solid corrosives may become highly aggressive upon contact with moisture or perspiration, wetted-solid protocols are applied in the metal corrosion test and considered in biological assessments.
Industrial Formulations and Electrolytes
Battery electrolytes, pickling and etching solutions, electroplating baths, descaling agents, acidic and alkaline cleaning formulations, and other multicomponent industrial products are submitted for classification, Safety Data Sheet authorization, and packaging compatibility verification. For mixtures, the evaluation considers both test data on the mixture itself and bridging principles applied to similar tested formulations.
Testing Items
Typical corrosivity test programs for hazardous chemicals include the following items:
- Skin corrosion potential (in vitro screening and, where necessary, confirmatory in vivo assessment)
- Serious eye damage and eye irritation potential
- Corrosion rate to steel and aluminum surfaces at 55 °C
- Assignment of packing group I, II, or III for Class 8 dangerous goods
- pH measurement and acid or alkali reserve (buffering capacity)
- Compatibility of the chemical with packaging materials, linings, coatings, and gaskets
Testing Methods
Physicochemical Screening: pH and Buffering Capacity
Testing typically begins with pH determination and measurement of acid or alkali reserve, commonly performed in accordance with OECD Test Guideline 122. Substances with pH ≤ 2 or ≥ 11.5 combined with high buffering capacity are generally presumed to be corrosive and may be classified as such without additional biological testing, provided the classification is not contradicted by other evidence.
In Vitro Skin Corrosion Tests
Validated in vitro methods are the primary tools for assessing skin corrosion. Under OECD Test Guideline 431, reconstructed human epidermis (RhE) models consisting of functional epidermal tissue are exposed to the test substance for defined periods, typically 3 minutes and 1 hour, after which cell viability is measured colorimetrically. A viability below the classification threshold indicates corrosion, and certain models allow discrimination among GHS sub-categories 1A, 1B, and 1C. The transcutaneous electrical resistance (TER) test under OECD Test Guideline 430 uses excited rat skin discs, where a sharp drop in electrical resistance signals loss of barrier integrity, while the in vitro membrane barrier test under OECD Test Guideline 435 monitors the time required for the chemical to penetrate a synthetic membrane and trigger a colorimetric detection system.
In Vivo Dermal Testing
Where in vitro results are inconclusive or negative but suspicion remains, a confirmatory in vivo test may be performed under OECD Test Guideline 404 using a sequential single-animal design. Exposure durations of 3 minutes, 1 hour, and 4 hours are applied with observation for up to 14 days, and testing is discontinued immediately if corrosive effects are observed. Regulatory authorities strongly encourage this test only as a last resort within the integrated testing strategy.
Eye Damage and Eye Irritation Methods
For serious eye damage, in vitro methods such as the bovine corneal opacity and permeability test (OECD Test Guideline 437), the isolated chicken eye test (OECD Test Guideline 438), and reconstructed human cornea-like epithelium models (OECD Test Guideline 492) are widely accepted for identifying chemicals that cause irreversible ocular damage and for supporting sub-classification decisions.
Metal Corrosion Test
The transport-specific metal corrosion test described in Section 37 of the UN Manual of Tests and Criteria evaluates whether a substance corrodes steel or aluminum at a rate exceeding 6.25 mm per year. Polished coupons of reference carbon steel (S235JR+CR or equivalent) and non-clad 7075-T6 or AZ5GU-T6 aluminum are exposed to the liquid substance, or to the wetted solid, at 55 °C for one week. The corrosion rate is calculated from coupon mass loss using the relationship: corrosion rate (mm/year) = (8.76 × 10^4 × W) / (A × T × D), where W is mass loss in grams, A is exposed area in square centimeters, T is exposure time in hours, and D is metal density in grams per cubic centimeter. A result exceeding 6.25 mm per year on either metal supports classification as Class 8 with packing group III, unless skin corrosion data justify a more severe packing group.
Packing Group Criteria for Class 8
Assignment of packing groups for Class 8 substances is based on tissue destruction data and observation periods:
- Packing group I: destruction of intact skin within 3 minutes or less of exposure, observed within 60 minutes
- Packing group II: destruction of skin after more than 3 minutes but not more than 60 minutes of exposure, with effects observed within 14 days
- Packing group III: destruction of skin after more than 60 minutes but not more than 4 hours of exposure with effects observed within 14 days, or a corrosion rate exceeding 6.25 mm per year on steel or aluminum at 55 °C
Material Compatibility and Durability Studies
Beyond regulatory classification, corrosion behavior is characterized using recognized material testing standards. ASTM G31 covers laboratory immersion corrosion testing of metals, ASTM B117 and ISO 9227 describe salt spray exposure for evaluating coatings and finished components, and ISO 175 and ASTM D543 address the resistance of plastics to chemical reagents. These studies support packaging selection, liner and gasket design, and long-term storage safety assessments.
Applications
Corrosivity test results support a wide range of regulatory and engineering decisions:
- Dangerous goods classification and assignment of UN proper shipping names and packing groups for Class 8 substances under the UN Model Regulations, IMDG Code, IATA DGR, ADR, and applicable national transport rules such as 49 CFR
- Selection of UN-certified packaging, including the X, Y, or Z packaging level corresponding to packing groups I, II, and III, and verification of compatibility with inner receptacles and closures
- GHS classification and labelling, and preparation of Safety Data Sheets covering hazard identification, physicochemical properties, stability, and transport information, in line with the GB 30000 series, GB/T 16483, and comparable international formats
- New chemical notification, registration, and regulatory review submissions where corrosivity data are required endpoints
- Workplace safety programs, including storage segregation, personal protective equipment selection, and emergency response planning
- Research, development, and quality control of chemicals, packaging systems, coatings, and corrosion-resistant materials
FAQ
What qualifies a chemical as a Class 8 corrosive substance?
A chemical is classified as Class 8 if it causes full-thickness destruction of intact skin tissue upon contact for up to 4 hours, or if it corrodes steel or aluminum at a rate greater than 6.25 mm per year at 55 °C. Either criterion is sufficient for classification, and the packing group is assigned according to exposure time, observation period, or measured corrosion rate.
Can pH alone determine corrosive classification?
Not by itself. Extreme pH values of ≤ 2 or ≥ 11.5 are strong indicators, but classification also depends on acid or alkali reserve. Substances with extreme pH and significant buffering capacity may be classified as corrosive without further testing, whereas low-buffer systems may require biological evaluation to confirm or rule out corrosion.
Is animal testing still required?
In most cases, no. Validated in vitro methods under OECD Test Guidelines 431, 430, 435, 437, 438, and 492 are accepted as primary tools for identifying corrosive substances. In vivo testing under OECD Test Guideline 404 is reserved for situations where in vitro data are inconclusive and is performed using a humane sequential design with the minimum number of animals.
When is the metal corrosion test necessary?
The steel and aluminum corrosion test is relevant when skin and eye data are negative or unavailable, but the chemical's chemistry suggests possible metal attack, such as with certain amines, salts, or halogenated compounds. Substances shown to be corrosive to skin are automatically Class 8, while those corrosive only to metals are assigned packing group III.
How much sample is needed and how long does testing take?
In vitro skin corrosion studies typically require a small volume of liquid or a few grams of solid per exposure condition and can be completed within one to two weeks. The metal corrosion test requires sufficient sample to fully immerse or wet multiple metal coupons and generally runs for approximately two weeks, including coupon preparation, the one-week exposure at 55 °C, and post-test mass-loss analysis. Accredited third-party laboratories can provide precise sample quantities and timelines based on the specific test program.
How do corrosivity results affect packaging and shipping documents?
The packing group determines the minimum UN packaging specification level, with packing groups I, II, and III corresponding to X, Y, and Z performance levels respectively. Results also feed into the hazard class, UN number selection, and hazard statements recorded on transport documents, labels, and Safety Data Sheet sections covering hazard identification, handling and storage, and transport information.