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| GB/T 18590-2025 | English | 320 |
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Corrosion of metals and alloys - Guidelines for the evaluation of pitting corrosion
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| GB/T 18590-2001 | English | 105 |
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Corrosion of metals and alloys -- Evaluation of pitting corrosion
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GB/T 18590-2025: Corrosion of metals and alloys - Guidelines for the evaluation of pitting corrosion
---This is an excerpt. Full copy of true-PDF in English version (including equations, symbols, images, flow-chart, tables, and figures etc.), auto-downloaded/delivered in 9 seconds, can be purchased online: https://www.ChineseStandard.net/PDF.aspx/GBT18590-2025
GB
NATIONAL STANDARD OF THE
PEOPLE’S REPUBLIC OF CHINA
ICS 77.060
CCS H 25
Replacing GB/T 18590-2001
Corrosion of metals and alloys - Guidelines for the
evaluation of pitting corrosion
(ISO 11463.2020, MOD)
Issued on: AUGUST 29, 2025
Implemented on: MARCH 01, 2026
Issued by. State Administration for Market Regulation;
National Standardization Administration.
Table of Contents
Foreword... 3
Introduction... 5
1 Scope... 6
2 Normative references... 6
3 Terms and definitions... 6
4 Identification and observation of pits... 6
4.1 Preliminary low-magnification visual inspection... 6
4.2 Optical microscope observation of pit size and shape... 7
4.3 In-situ non-destructive testing... 8
4.4 Ex-situ observation techniques... 9
5 Pitting severity... 11
5.1 Weight loss... 11
5.2 Pitting depth measurement... 11
6 Evaluation of pitting corrosion... 13
6.1 Overview... 13
6.2 Standard chart method... 13
6.3 Metal penetration method... 15
6.4 Statistical method... 16
6.5 Loss of mechanical properties... 18
7 Test report... 18
8 Additional information... 19
References... 20
Corrosion of metals and alloys - Guidelines for the
evaluation of pitting corrosion
1 Scope
This document provides guidelines for selecting procedures for identifying and
observing pits, evaluating pitting corrosion and pit growth rates.
This document is applicable to the evaluation of pitting corrosion in metals and alloys
after pitting has occurred.
2 Normative references
This document has no normative references.
3 Terms and definitions
This document has no terms and definitions that need to be defined.
4 Identification and observation of pits
4.1 Preliminary low-magnification visual inspection
4.1.1 The corroded metal surface can be visually inspected, with or without a low-
magnification magnifying glass, to determine the extent of corrosion and the apparent
location of pits. It is usually recommended to photograph the corroded surface for
comparison with a clean surface after removing corrosion products or with a new,
unused sample.
4.1.2 If the metal specimen is exposed to an unknown environment, the composition of
the corrosion products may be valuable in determining the cause of corrosion. It is
recommended to follow the recommended procedure for removing particulate corrosion
products; these removed products should be preserved for future identification.
4.1.3 To fully expose the pits, a cleaning procedure should be used to remove corrosion
products. For lightly adhering corrosion products, rinsing with water followed by light
mechanical cleaning is sufficient. For more strongly adhering products, chemical
cleaning is required. GB/T 16545[1] provides a series of chemical cleaning processes.
To ensure that the base material is not corroded, preliminary tests should be conducted.
4.2 Optical microscope observation of pit size and shape
4.2.1 Observe the cleaned metal surface to determine the approximate size and
distribution of the pits. Then, use a low-magnification microscope (approximately 20×)
for more detailed observation. Pits may have various sizes and shapes. Visual inspection
of the metal surface can reveal circular, elongated, or irregular openings; however, it is
rarely possible to accurately observe the extent of corrosion below the surface of the
pits. Therefore, cross-sectional analysis of the pits is usually required to determine their
actual shape. Figure 1 shows several common cross-sectional shapes of pits.
f) Microstructure orientation - Horizontal type g) Microstructure orientation - Vertical type
Figure 1 -- Different cross-sectional shapes of pits
4.2.2 Counting the number of pits with a microscope to determine the pit density is
difficult. However, using a plastic grid makes it easier. Place a grid with square meshes
of 3 mm ~ 6 mm side length on the metal surface; record the number of pits in each
square mesh. Then move the grid sequentially until the entire surface has been observed.
This method reduces eye strain, because the field of view can be observed without
worrying about missing significant areas. Magnifying significant areas also reduces eye
strain. Another method is to mount the specimen on an x-y stage; measure the number
and spatial distribution of the pits. Combined with optical depth measurement where
applicable, this can determine the number, depth, spatial distribution of the pits.
4.2.3 In situations where optical observation is limited, advanced optical microscopy
techniques such as infinite-focus microscopes and laser confocal microscopes can be
used, to obtain three-dimensional images of the pit surface. This is most suitable for
Figures 1a) ~ 1c), but not for undercut types. This measurement can be used to observe
surface features and quantify surface roughness, pit depth, surface profile.
4.2.4 For metallographic observation, a representative metal surface containing pits
shall be selected to prepare metallographic specimens. If observing corrosion products
along the cross-section, the surface can be fixed with an embedding compound before
sectioning, if necessary. Microscopic observation can determine whether the pits are
related to inclusions or microstructure, or determine whether these pits are true pits or
metal loss caused by intergranular corrosion or dealloying corrosion.
4.3 In-situ non-destructive testing
4.3.1 Overview
Many techniques have been developed to detect cracks or voids on metal surfaces
without damaging the material (see reference [2]). Compared to the methods mentioned
earlier, these methods are less effective in locating and determining the shape of the pits,
but they are more suitable for on-site applications because they are often used for in-
situ measurements.
4.3.2 Radiography
Radiation such as X-rays can penetrate objects. The intensity of the transmitted
radiation varies with the thickness of the material. Defects can be detected if they cause
a change in X-ray absorption. Detectors or films can be used to provide the morphology
of internal defects. The detection of metal thickness mainly depends on the effective
energy output. The pits shall be deep enough to be 0.5% of the metal thickness to be
detected; care should be taken to ensure that the pits are not confused with pre-existing
voids.
4.3.3 Electromagnetic methods
4.3.3.1 Eddy currents can be used to detect structural defects and irregularities in
conductive materials. When a specimen is exposed to a changing magnetic field
generated by a coil carrying alternating current, eddy currents are induced in the
specimen, which in turn generate their own magnetic field. The magnetic field
generated by defective material differs from that of a defect-free reference material,
meanwhile appropriate detection instruments are needed to determine these differences.
4.3.3.2 Magnetic induction in ferromagnetic materials is another applicable method.
Discontinuities in the magnetic field across the cross-section lead to the formation of a
leakage magnetic field on the surface of the material. Ferromagnetic powder is placed
Non-contact methods record the same type of information, typically using laser-based
optical methods such as infinite focus microscopy, which do not require direct physical
contact with the sample surface. These techniques derive 3D surface profiles by
accumulating images at different optical focal planes. Using white light interferometry,
which compares the phase difference between light reflected from the sample surface
and light reflected from a reference mirror, 3D surface profiles can be obtained by
recording surface morphology path length differences. Laser confocal microscopy can
also provide similar information.
The disadvantage of these techniques is that they can only characterize information that
is optically detectable, which are mainly suitable for the types of pits shown in Figures
1a) ~ 1c) (see also 4.2.3).
5 Pitting severity
5.1 Weight loss
Weight loss measurement is generally not recommended for measuring pitting severity,
unless uniform corrosion is very slight and pitting is quite severe. If uniform corrosion
is significant, meanwhile pitting has little effect on the weight loss of the metal, then
pitting damage cannot be accurately assessed by weight loss methods. In any case,
weight loss only provides the total metal loss due to pitting, but does not provide
information on pit density and depth. However, weight loss shall not be ignored in any
case, as it can be very valuable. For example, weight loss combined with observation
of the pitted surface is sufficient to evaluate the pitting resistance of an alloy in
laboratory tests. Weight loss may also help to detect metal loss existing below the
surface.
5.2 Pitting depth measurement
5.2.1 Metallographic method
The depth of the pits can be determined by vertically sectioning a pre-selected pit,
mounting the cross-section using metallographic techniques, then polishing the surface
for measurement. A better or alternative method is to section the sample slightly off-
center from the pit and then slowly grind down to the cross-section where the pit
appears. Precisely sectioning through the center of the pit is difficult and may miss the
deepest part. The depth of the pit needs to be measured on a flat, polished surface using
a microscope with a calibrated eyepiece. This method is very accurate, but it requires
good operational skills and judgment in selecting the pit, as well as good technique in
sectioning the pit. Its limitations include being time-consuming, potentially failing to
select the deepest pit, the section not being at the deepest point of the pit. This technique
also results in the destruction of the specimen.
5.2.4 Microscopic method
5.2.4.1 This method is particularly valuable when the pit is very narrow or the
instrument's probe is difficult to insert into the pit. This method can be used, as long as
light can be focused on the bottom of the pit. This method is not suitable for the situation
shown in Figure 1e).
5.2.4.2 A metallographic microscope with a magnification range of 50× ~ 500× and a
calibrated fine adjustment knob (e.g., 1 division = 0.001 mm) is used. If there is no fine
adjustment knob, a dial indicator can be mounted on the microscope, to show the
movement of the stage relative to the microscope body.
5.2.4.3 Center a single pit on the metal surface under a low-magnification (50×)
objective lens. Increase the magnification until the pit occupies most of the field of view.
Focus on the edge of the pit, first roughly and then finely adjusting the focus. Record
the initial reading of the fine adjustment knob. Focus on the bottom of the pit using the
fine adjustment knob and record the reading. The difference between the initial and
final readings of the fine adjustment knob is the depth of the pit.
5.2.4.4 Repeat step 5.2.4.3 for each pit to determine the pit depth distribution.
Alternatively, many modern instruments are equipped with software that automatically
measures pit depth from optical microscope images.
5.2.4.5 One method derived from this technique is the use of an interference microscope.
A beam of light is split into two; one part is projected onto the specimen, whilst the
other onto the surface of a reference mirror. The light reflected from these two surfaces
is recombined to form interference fringes, providing a topographic map of the
specimen surface. These fringes can be used to measure vertical deviations on the metal
surface. However, this method is limited to shallower pits with depths less than 25 μm,
as the increasing number of fringes makes counting difficult.
6 Evaluation of pitting corrosion
6.1 Overview
Several methods can be used to describe the pitting corrosion, providing a quantitative
expression of its significance or for predicting the lifespan of a material. This chapter
will describe some of the more commonly used methods; typically, using any single
method alone is insufficient.
6.2 Standard chart method
Note. See reference [4].
......
Source: Above contents are excerpted from the full-copy PDF -- translated/reviewed by: www.ChineseStandard.net / Wayne Zheng et al.
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