June 2016 Archives

"Hexavalent chromium" is designated as harmful substances by Japanese laws related to environmental conservation, such as Water Pollution Control Act, Soil Contamination Countermeasures Act, as well as those related to industrial health and safety. These laws and legislations strictly regulate its application and discharge methods along with the threshold limit value for discharge.

In order to comply with the restriction of the use of certain hazardous substances in electrical and electronic equipment (RoHS) to be enforced in July 2006, the hexavalent chromium content (ppm) is a serious problem for rubber, plastic, paint, chemical film, and ceramic products.

Hexavalent chromium

What is hexavalent chromium? Chromium is a metal element with atomic number 24, melting point of 1,905°C, and specific weight of 7.1 g. Because many of its compounds are colored, the name was derived from "chroma (Greek word meaning 'color')".

In addition to being used as decorative and industrial (hard) chrome plating, chromium is used in metal alloys, such as high-speed steel, stainless steel, nichrome, and KS magnet steel.

Chromium metal is not water-soluble. However, chromium compounds are relatively soluble in water. Chromium in chromium compounds is a positive ion carrying two to six positive charges. The number of electrical charges varies by compounds.

In the case of chromium plating solution, for example, chromic acid (H₂CrO₄) is generated by dissolving commercially available chromic anhydride into water. The reaction occurs as follows:

Chromic acid is dissociated into hydrogen ion and chromate ion in the solution.

Direct current electrolysis produces hydrogen gas (H₂) at the cathode. Chromate ion (CrO₄^2-) will be attracted to the anode and turned into hexavalent chromium ion (Cr⁶⁺) and oxygen (O₂) by a catalytic action at the anode. Hexavalent chromium becomes metallic chromium after receiving electrons in stages.

As we have seen, hexavalent chromium serves an important role as a supply source of chromium metal in plating solution. However, metallic chromium, which is used for plating films, does not contain hexavalent chromium.

Now, let's look at the chromate treatment after applying zinc plating. Electro-galvanizing and hot-dip galvanizing are used for construction materials, structures, ship equipment and more to prevent corrosion. However, zinc is known to form zinc oxide (white rust) by reacting with oxygen in the air.

To minimize this unfavorable character, the chromate treatment came in handy. The chromate treatment is a process where galvanized products are dipped into chromate treating solution, consisting primarily of chromic acid, to form an anticorrosive chromate film (such as CrO₃, Cr₂O₃, and nH₂O) containing hexavalent chromium on the plating surface. The higher content of hexavalent chromium in a film results in superior anticorrosive properties. Even if the chromate film has been damaged for some reason, the surrounding hexavalent chromium works great in repairing its property on its own.

There are various types of chromate films: bright chromate, colored chromate, black chromate, green chromate, and more. The content of hexavalent chromium differs for each of them. In general, it should be in the range between 20 and 120 mg/m².

Although you can easily identify heavy metals using fluorescence X-ray, atomic absorption, and ICP emission spectrometric analyses, materials like cyanogen (CN-) and hexavalent chromium (Cr6+) require another analysis method. The method is called absorption photometry where you compare the color density of a substance against the object substance by coloring a pretreated sample with an indicator.

(1) Measurement principle

[Fig.1] shows the principle of density measurement by absorption photometry. This method is very similar to the colorimetric method where you compare a color against the standard color. Place a colored sample into the measurement cell as shown in [Fig.1] (a). Transmit the light from the light source through the filter to convert it into light in the specified wavelength (Io). During the transmission, part of the light will be absorbed. The transmitted light (It) is then converted into an electric signal by the photoelectric cell and displayed as transmittance or absorbance.

If you use a prism instead of a filter, the light wavelength can be selected from a wider range (spectroscopy). As shown in [Fig.1] (b), the maximum absorption wavelength can be found by measuring the absorbance while changing the wavelength for the colored sample. Use the light in this wavelength and measure absorbance of the standard solution whose concentration rate is already known. Then, prepare the standard curve graph (c) in order to predict the concentration of an unknown sample.

(2) Equipment overview

[Fig.2] shows the appearance of an absorptiometer.

(3) Characteristics

To determine how much hexavalent chromium is contained in a certain product, cut off a part of the product with a certain surface area and extract hexavalent chromium by pure water. Dilute or concentrate the extraction liquid to a proper concentration. Add the coloring indicator and measure absorbance to calculate the concentration of hexavalent chromium.

(1) Measurement principle

ICP (Inductively Coupled Plasma) emission spectrometric analysis uses the inductively coupled plasma flame, at higher temperature (6,000 to 8,000K) than the burner used for the atomic absorption analysis (no more than 3,000°C), to cause dissociative excitation of measured substances in a broader range. The qualitative analysis will be performed by the wavelength position of atomic spectral lines emitted. The quantitative analysis will be performed by the emission intensity.

(2) Equipment

The analysis equipment is comprised of the followings: emission unit (for vaporizing sample and emitting light), spectroscopic unit (for dispersing emission spectral lines), and photometry unit (for measuring spectral lines). [Fig.1] illustrates the structure and mechanism of an ICP discharge tube. The plasma discharge tube has a three-layered structure made of quartz. A coil is wired on the top. A high-frequency magnetic field is induced by passing high-frequency current through the coil. If you add argon gas here, it will generate high-temperature plasma flame. Place the sample prepared a mist form by a nebulizer (atomizing device) onto a flow of argon gas. Once the gas has flown into the plasma flame, light emission occurs after vaporization, atomization, and excitation processes. Analyze this emission spectrum by the spectroscopic and photometry units.

[Fig.2] illustrates the analysis equipment system.

(3) Characteristics

The ICP emission spectrometric analysis can identify a lot more elements than the atomic absorption analysis. In addition to heavy metals, elements such as C (carbon), P (phosphorus), and S (sulfur) can be analyzed as well. Because this method is capable of analyzing multiple elements simultaneously in a single sample, it is used in a variety of fields, including food, textile, environment, and semiconductor industries in addition to steel and non-ferrous metal industry.

(1)Measurement principle

This is an official analysis method adopted by JIS as well. By placing atoms in a high-temperature flame (some methods do not involve the flame), the outermost electron of all the electrons surrounding the nucleus will be excited to a higher energy level. When they return to the ground electronic state after a short period, each element emits light at a characteristic wavelength of the corresponding atom. This process is called "atomic emission".
On the other hand, when you emit specific light from the outside to this atom, the light will be absorbed by the atom. This process is called "atomic absorption". The degree of light being absorbed (absorbance) is an indicator of the corresponding atom's concentration.

(2)Measurement device

The figure here is an example of the optical system for an atomic absorption photometer. Use an appropriate type of a hollow-cathode lamp (HCL) for the atom absorption wavelength you are going to measure. The deuterium lamp (D2) is used for background correction. Substances such as hydrogen, acetylene, and nitrous oxide are used for the heat source of the burner (B) to bring atoms to the excited state. Inject the sample in a mist form into this burner. The light passed through the burner flame is converted into an electric signal by the photomultiplier tube (PMT), which will be displayed and printed out as the concentration data.

(3)Characteristics

Although this method can analyze most of the metallic elements, a pretreatment process is required prior to analysis. Unlike the fluorescence X-ray analysis introduced in the previous volume, this method is not designed for analyzing samples in the solid state.
The preparation method may vary product to product based on the materials, but it is necessary to prepare the liquid sample for measurement by dissolving the target element using strong acid like "agua regia" and then diluting it at an appropriate multiplying factor. Since the sample's concentration is measured in units of single-digit ppm, the technological skills of diluting samples by adding pure water will determine the degree of errors.