August 2009 Archives

#009 Hydrogen Embrittlement - Baking Process - 1

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For plating products with hydrogen embrittlement risks, baking processes are performed as means of dehydrogenation process. The baking process typically involves heating the products in furnaces at 190~220 deg.C to purge the hydrogen. The heating time will depend on the plating type, pre-plating process used, deposition thickness, alloy type, and the condition of the base metal.

(1)Pre-process and effects of baking

[Fig. 1] and [Fig. 2] below show the effects of dehydrogenation by baking on material samples with zinc plating where one sample with no hydrogen occlusion, and the other with hydrogen occlusion during the pre-process. Zinc plating is used as a representative more likely to cause hydrogen embrittlement.

[Fig. 1] shows zinc plated samples pre-processed in a non hydrogen occluding acid bath pickling. The sample does occlude some hydrogen through the zinc deposition layer but one hour of baking will effectively dehydrogenate the sample in all types of zinc baths as seen.

[Fig. 1] Non hydrogen occluding pickling used

[Fig.2] shows samples that occluded large amounts of hydrogen during the pre-process pickling as well as the zinc plating process. As can be seen in the graph, little hydrogen is purged from the alloy even after long baking time.

[Fig. 2] Hydrogen occluding pickling used

From the above, it can be understood that prevention of hydrogen occlusion is important during the pickling process, or some type of dehydrogenation process will be required before the baking process if the work had already occluded some hydrogen during pickling. Effective hydrogen occlusion prevention includes use of proper inhibitors, and in the latter case, immersion in high temperature alkaline degreasing bath. As can be seen in the graphs, ammonium chloride zinc bath is effective in preventing hydrogen embrittlement.

#008 Hydrogen Embrittlement - Plating Embrittlement

[Fig.1] shows a listing of hydrogen embrittlement rates of various plating processes. It is easily imaginable that possibilities of hydrogen embrittlement exists for electro-plating processes, which uses the work piece as a cathode electrode for the electrolysis. However, non-electrolysis process such as electroless nickel plating could also cause hydrogen embrittlement as well. Here, the hydrogen generation is thought to be caused by reducing agents used in place of the direct current used for electro-plating.
Also, in chrome plating where non dissolving anodes such as lead and graphite are used, unlike other plating processes where dissolving anodes (zinc, nickel, etc.), vigorous electrolysis of water takes place during redox of chromate ions dissolved in the plating bath in order to obtain the chromium metal. This vigorous electrolysis causes hydrogen generation.

[Fig.1] Hydrogen embrittlement rates of various plating processes

From [Fig.1] above, it can be seen that acidic plating baths such as nickel plating, nickel alloy plating, and chloride zinc plating have very low hydrogen embrittlement rates, but alkaline plating baths such as zinc cyanide bath and copper cyanide bath are more likely to cause hydrogen embrittlement.
It is considered, in general, that most of the electrical current applied is used for electrolysis of water resulting in high hydrogen generation with the alkaline baths due to low cathode current efficiency, as opposed to acidic baths where the current is mostly used for redox of metal ions due to high cathode current efficiency.

Hydrogen embrittlement rates of zinc plating in cyanide bath (alkaline) and chloride bath (acidic) are shown in [Fig.2] below.

[Fig.2: Hydrogen embrittlement rates of zinc plating

#007 Hydrogen Embrittlement - Dehydrogenation prior to Plating

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The occluded hydrogen during pickling process is thought to be retained near the steel surface, and is recommended that the hydrogen be released prior to plating layer is deposited. The hydrogen is slowly released by simply letting the part sit out in the air, as shown in [Fig.1] below.

[Fig.1] Hydrogen embrittlement rate and sitting-out time duration

Diffusion movement of hydrogen accelerates at higher temperatures, so the dissipation time of occluded hydrogen can be shortened by heating. Normally, an alkaline wash process follows a pickling process, and relatively high temperature of 60~70 deg.C for this wash process is advantageous for the hydrogen dissipation. A test result is shown in [Fig.2] below.

[Fig. 2] Alkaline wash after pickling and hydrogen embrittlement

In actual plating work practices, it is difficult to setup the alkaline wash stage for the purpose of dehydrogenation due to variations of rust and scales on the subject metal and wash process time.

For steel parts that are subjected to long pickling processes, a separate dehydrogenation process (baking) is required. Although it is more effective to bake for long durations at high temperatures, this will cause oxide layers to form on the steel surface, requiring a pickling again. So the baking takes place at i.e. 200 deg.C for 30 minutes, as shown in [Fig.1] above.

For example, baking is typically performed after zinc plating, because this is intended to release the hydrogen occluded during the zinc plating. But, the hydrogen occluded during pickling needs to be released before plating or else the zinc coating will prevent sufficient emission.

#006 Hydrogen Embrittlement - Inhibitor

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Pickling process is performed to remove rust and scales from surfaces of steel parts, and inhibitors have been in use to prevent excess dissolution of the steel parts. In fact, the inhibitors, in addition for prevention of excess steel dissolution, act to prevent hydrogen embrittlement as well.
Following chemical reactions occur in hydrochloric acid pickling without inhibitor addition.

Fe2O3 + 6HCL → FeCL3 + H2O(Dissolution reaction of steel rust.)
Fe + 2HCL → FeCL2 + 2H(Dissolution reaction of steel alloy)

The hydrochloric acid will dissolve rust and scales on steel surface and forms chlorides but will not generate hydrogen (see Dissolution reaction of steel rust, above). Further into the process, base steel surface is exposed as the rust/scale layer is dissolved and the steel will begin to dissolve (see Dissolution reaction of steel alloy, above), causing hydrogen formation. Here the steel alloy begins to occlude the hydrogen in atomic state and hydrogen embrittlement will result.
The inhibitor prevents this dissolution of the steel alloy thus preventing the formation of hydrogen. The inhibitor prevents the hydrogen formation by selectively adhering to the exposed steel surface to prevent the acid solution from contacting the surface, thus preventing the hydrogen from forming.

[Fig.1]Action of the inhibitor

As seen above, the use of inhibitors in pickling are effective in hydrogen embrittlement prevention. However, since chemical composition details are unclear for many commercially available inhibitors, it is advisable to test them before actual use.
For sulfuric acid, hydrochloric acid, and phosphate pickling baths, diethylthiourea and dibutylurea are well known.
The chemical compositions of commercial inhibitors are not published, but the following are know to be effective for hydrogen embrittlement suppression, according to some published material.

(1)Organic compounds containing nitrogen, amines
(2)Organic compounds containing oxygen, carboxylic acid, etc.
(3)Organic compounds containing phosphor, thiourea, etc.
(4)Acetylene compounds, propargylalcohol, etc.

There is, however, a problem with the commercial inhibitors. Since the chemical composition details are unknown, proper concentration management becomes difficult. At low concentration levels, effective prevention of hydrogen embrittlement cannot be expected, and on the other hand at high concentration levels, the agent that may be carried over may affect the next process negatively.

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