What is ISO 9223 and what does it classify?

The ISO 9223 standard (current version: ISO 9223:2012) provides an international framework for classifying, determining, and predicting the corrosiveness, or corrosion potential, of atmospheres in which metals and alloys are located. By categorizing atmospheres according to their corrosive severity, the standard provides a common language for material and protection system selection.

The standard determines the corrosiveness category using two main approaches: experimental determination based on measuring the corrosion loss of standard samples in the first year, and estimation based on knowledge of local environmental conditions. This dual approach makes category estimation possible through both direct measurement and available environmental data.

This content is for informational purposes only and provides general technical information; a corrosiveness assessment for a specific facility or structure requires a qualified engineer's technical assessment and appropriate accredited testing processes.

  • Objective: To classify the corrosion potential of the atmosphere.
  • Current version: ISO 9223:2012
  • Two approaches: experimental determination and prediction based on environmental conditions.
  • It provides a common reference for the selection of materials and protection systems.

Corrosion Categories: C1 to CX

ISO 9223 defines six categories for atmospheric corrosivity. These categories are determined by the first-year corrosion loss (loss in mass or thickness) of standard metal samples and form an increasing scale from low corrosivity to extreme corrosivity.

The CX category was added in the 2012 revision of the standard and represents the most challenging conditions, such as extreme tropical and marine atmospheres where coastal and industrial influences coexist. This addition addresses the need to adequately describe environments above C5 in the previous version.

  • C1: very low corrosiveness (very low pollution environments such as dry, heated indoor spaces)
  • C2: low corrosiveness (low-pollution rural and some indoor environments)
  • C3: moderate corrosiveness (urban and industrial environments with moderate levels of sulfur dioxide)
  • C4: High corrosiveness (industrial and coastal environments with high chloride or pollutant loads)
  • C5: very high corrosiveness (high humidity, aggressive industrial and marine atmospheres)
  • CX: Extreme corrosiveness (added in 2012; extreme marine and tropical/industrial environments)

Environmental Factors Determining the Categories

ISO 9223 defines the determining environmental factors that drive atmospheric corrosion. Foremost among these is the wetness time (TOW), derived from the temperature-humidity combination; this is the period during which an electrolytic water film exists on the metal surface and electrochemical corrosion is active.

The second determining factor is sulfur dioxide (SO2) pollution; this refers to the precipitation of atmospheric sulfur compounds that significantly affect the corrosion rate in industrial and urban environments. The third factor is chloride (Cl-) precipitation; this significantly accelerates corrosion, especially in coastal atmospheres, depending on the salinity of the air.

By evaluating these three factors (wetting time, sulfur dioxide precipitation, and chloride precipitation) together, the corrosiveness category of a medium can be estimated from environmental conditions. The measurement methods for these parameters are discussed in the related ISO 9225 standard.

  • Time to Wet (TOW): the time the electrolyte water film remains on the metal surface.
  • Sulfur dioxide (SO2) precipitation: an indicator of industrial and urban pollution.
  • Chloride (Cl-) precipitation: a defining characteristic in coastal and saline environments.
  • These factors together form the basis for category prediction.

Relationship Between First-Year Corrosion Rate and Category

In ISO 9223, corrosivity categories are defined according to the corrosion loss of standard samples (typically low-carbon steel, zinc, copper, and aluminum) after one year of exposure. Conceptually, the higher the first-year corrosion loss, the higher the corrosivity category of the medium; C1 corresponds to the lowest first-year loss, and CX to the highest.

The standard defines first-year corrosion loss ranges for each category based on the relevant metals. Using these numerical ranges for a specific application requires direct reference to the current standard text and a qualified engineer's technical assessment; precise numerical values are left at a conceptual level in this document.

ISO 9226 specifies how to experimentally determine corrosion loss in the first year, while ISO 9224 addresses how to handle long-term behavior.

  • The category is based on the corrosion loss of standard samples in the first year.
  • High loss = high corrosivity category (C1 lowest, CX highest)
  • Standard metals: low carbon steel, zinc, copper, aluminum.
  • The current standard text should be used for precise range values.

Related Standards: ISO 9224, ISO 9225 and ISO 9226

ISO 9223 is not used alone, but in conjunction with a complementary family of standards. ISO 9224 provides guidance values for long-term corrosion behavior for corrosivity categories; it distinguishes between the average corrosion rate in the initial years and the steady-state corrosion rate after long-term exposure.

ISO 9225 specifies the measurement methods for environmental parameters used in corrosiveness assessment; these include wetting time and the measurement of settling rates of pollutants such as sulfur dioxide and chloride. ISO 9226 defines the experimental determination of the corrosion rate on standard samples, i.e., the measurement of corrosion loss, which forms the basis for category determination.

These four standards together form a comprehensive framework for the classification of atmospheric corrosivity (9223), long-term estimation (9224), measurement of environmental parameters (9225), and experimental corrosion rate determination (9226).

  • ISO 9224: long-term corrosion rate guideline values.
  • ISO 9225: Environmental parameter and pollutant measurement methods
  • ISO 9226: Experimental determination of corrosion rate in standard specimens.
  • The four standards together provide a comprehensive framework for corrosivity.

Relationship and Scope of Application of ISO 12944-2

Corrosion categories are also used in the ISO 12944 family of standards dealing with corrosion protection by paints. ISO 12944-2 classifies atmospheric environments into C1-CX corrosivity categories in accordance with ISO 9223, and these categories are considered as an input in the selection of protective coating systems.

This paper focuses on atmospheric corrosivity classification. The role of corrosivity category (C1-CX) in paint system selection is only touched upon at the input level; ISO 12944 paint protection is discussed on a separate page. Similarly, cathodic protection is a topic discussed on separate pages and is outside the scope of this page.

Corrosion category information provides a common basis for many engineering decisions, such as material selection, protection system design, assessment of expected service life, and maintenance planning.

  • ISO 12944-2 classifies atmospheric environments into C1-CX categories, consistent with ISO 9223.
  • Corrosion category is an input in the selection of a protection system.
  • ISO 12944 paint protection is discussed on a separate page.
  • Cathodic protection is outside the scope of this page and will be discussed on separate pages.

Scope Limitation and the Role of AES

AES is NOT a certification/approval body; the determination of corrosiveness category and corrosion test certification are the responsibility of separate accredited laboratories/organizations. Experimental determination of atmospheric corrosiveness category requires that measurements under ISO 9225 and ISO 9226 be carried out by laboratories with the relevant accreditation.

AES Innovation is a Type A inspection body operating in Türkiye under TÜRKAK accreditation, specifically in the field of electrical installations (6.1 Electrical Installations). This document has been prepared to share general and objective technical information on atmospheric corrosivity.

This content is for informational purposes only. When a corrosiveness assessment is required for a specific structure, facility, or equipment, it is recommended to refer to current standard texts and consult a qualified engineer for technical assessment.

  • AES is not an organization that provides certification or conducts testing on corrosivity/corrosion.
  • AES accreditation is only in the 6.1 Electrical Installation field.
  • Corrosion determination is the responsibility of separate accredited laboratories/organizations.
  • Current standards and technical assessments by qualified engineers are essential for evaluation.

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Frequently Asked Questions

How many corrosivity categories does ISO 9223 define?

ISO 9223 defines six categories: C1 (very low), C2 (low), C3 (medium), C4 (high), C5 (very high), and CX (extreme). The CX category was added in the 2012 revision of the standard.

What are the key environmental factors that determine the corrosiveness category?

There are three determining factors: wetness duration (TOW), sulfur dioxide (SO2) pollution precipitation, and chloride (Cl-) precipitation. Chloride is particularly decisive in coastal environments.

What does the CX category represent?

CX is an extreme corrosiveness category and represents the most challenging atmospheric conditions, where extreme marine and tropical/industrial influences coexist. It was added with ISO 9223:2012.

What is the relationship between ISO 9223 and ISO 9224, 9225, and 9226?

ISO 9223 defines the classification; ISO 9224 provides guideline values for long-term corrosion rate; ISO 9225 specifies methods for measuring environmental parameters and contaminants; and ISO 9226 defines the experimental determination of corrosion rate in standard samples.

How is the corrosiveness category related to ISO 12944?

ISO 12944-2 classifies atmospheric environments into C1-CX categories, consistent with ISO 9223. The corrosiveness category is an input in the selection of a protective coating system. ISO 12944 paint protection is discussed on a separate page.

Does AES perform corrosiveness category determination or certification?

No. AES is not a certification/approval body; the determination of corrosiveness category and corrosion test certification are the responsibility of separate accredited laboratories/organizations. AES accreditation is only in the 6.1 Electrical Installation field.