怎么去選擇表面處理電鍍

　　電鍍就是在緊固件上鍍上一層金屬鍍層來改變其表面性質,使其不易氧化腐蝕,鍍層金屬一般多采用耐腐蝕性的金屬。

　　電鍍不僅能增強緊固件的抗腐蝕性,還可增加硬度防止磨損,提高導電性、耐熱性,以及使表面更光滑美觀。

　　電鍍是利用電解作用使金屬或其它材料制件的表面附著一層金屬膜的工藝。下面簡要介紹一些緊固件常用的鍍層。

　　一、電鍍代號:

　　在說到壓鉚類緊固件的標識時,提過表面處理的后綴代號,這些代號代表了其表面的電鍍類型。

　　下面我們將緊固件中常用常見的一些表面處理方式都統計了份明細資料。如下表:

　　二、關于電鍍:

　　1、電鍍鋅

　　電鍍鋅是緊固件最常用的鍍層,外觀好看也比較便宜,有白鋅、藍鋅、彩鋅、黑鋅等色彩。與其它金屬鍍層相比鋅是相對便宜又易電鍍的一種金屬,然而呢它的防腐蝕性能一般,電鍍鋅中性鹽霧試驗在72小時之內,當然也有采用特殊封閉劑,可使中性鹽霧試驗達200小時以上,但價格比較貴是一般鍍鋅的5~8倍。

　　2、電鍍鎳

　　電鍍鎳的緊固件一般用于既要防腐蝕性高,又要導電性好的地方。電鍍鎳層在空氣中的穩定性很高,由于金屬鎳具有很強的鈍化能力,在表面能迅速生成一層極薄的鈍化膜,能抵抗大氣、堿和某些酸的腐蝕。電鍍鎳有優良的拋光性能,經拋光后其光澤可長期保持,且鎳鍍層的硬度較高可提高緊固件的耐磨性。

　　3、氧化

　　氧化發黑+涂油是工業緊固件很流行的鍍層,因為它最便宜并且在油耗盡之前看起來不錯。在有油狀態下其中性鹽霧試驗也只能達到3~5小時。發黑的緊固件扭矩—預緊力一致性也很差。如需提高,可以在裝配時在內處螺紋上涂抹油脂后再旋合。

　　4、電鍍鉻

　　在緊固件上用鉻鍍層一般是作為裝飾作用。鉻鍍層在大氣中很穩定,不易變色和失去光澤,硬度高耐磨性好。好的電鍍鉻緊固件與不銹鋼同樣昂貴,只是使用不銹鋼強度不夠時,才用鍍鉻緊固件代替,因此在防腐蝕性要求較高的工業領域很少使用。為了防止腐蝕,鍍鉻前應首先鍍銅和鎳。鉻鍍層可以承受650℃的高溫,但與電鍍鋅一樣存在氫脆問題。

　　三、電鍍標準及質量檢驗:

　　緊固件表面處理國家標準GB/T5267.1-2002螺紋緊固件電鍍層標準,該標準包括:GB/T5267.1-2002緊固件電鍍層,GB/T5267.2-2002緊固件非電解鋅片涂層兩個標準。本標準等同采用國際標準ISO4042-1999螺紋緊固件電鍍層標準。

　　緊固件的表面處理,主要目的是使緊固件提高防腐蝕能力,以增加緊固件使用的可靠性和適應性,主要衡量標準是耐腐蝕性,其次是外觀。

　　緊固件電鍍層的質量主要從以下幾個方面來判斷:

　　1、外觀檢查

　　緊固件表面應光滑、光澤度好無漏鍍層面,不得有臟污、孔隙、針孔、剝離、燒焦鍍層、灰暗、起皮、結皮狀況和明顯條紋以及麻點、黑色鍍渣、鈍化膜疏松、龜裂、脫落和嚴重的鈍化痕跡。

　　2、鍍層厚度

　　緊固件的鍍層厚度直接關系到它在大氣中的耐腐蝕強度,但是過于厚的話在安裝過程中反而會產生螺紋干涉,建議一般鍍層厚度為4~12um。

　　熱浸鍍鋅標準的平均厚度為54 um(稱呼徑≤3/8為43 um),最小厚度為43 um(稱呼徑≤3/8為37 um)。

　　3、鍍層分布

　　采用不同的沉積方法,鍍層在緊固件表面上的聚集方式也不同。電鍍時鍍層金屬并不是均勻地沉積在外周邊緣上,轉角處獲得較厚鍍層。在緊固件的螺紋部分,最厚的鍍層位于螺紋牙頂,沿著螺紋側面漸漸變薄,在牙底處沉積最薄。

　　而熱浸鍍鋅正好相反,較厚的鍍層沉積在內轉角和螺紋底部,機械鍍的鍍層金屬沉積傾向與熱浸鍍相同,但是更為光滑而且在整個表面上厚度要均勻得多。

　　4、氫脆

　　緊固件在加工和處理過程中,尤其在鍍前的酸洗和堿洗以及隨后的電鍍過程中,表面吸收了氫原子沉積的過程中產生氫。當緊固件擰緊時,氫朝著應力最集中的部分轉移,引起壓力增高到超過它的強度并產生微小的表面破裂,氫滲入到新形成的裂隙中去。這種壓力-破裂-滲入的循環一直繼續到緊固件斷裂。通常發生在第一次應力應用后的幾個小時之內。為了消除氫脆的威脅,緊固件要在鍍后3小時內加熱烘烤,以使氫從鍍層中滲出,通常在200℃左右的溫度下進行,處理時間根據其要求的抗拉強度來決定。

　　由于機械鍍鋅是非電解質的,這實際上消除了氫脆的威脅,因此熱浸鍍的緊固件很少發生氫脆。

　　四、緊固件的熱處理:

　　熱處理是將緊固件通過一定的加熱,保溫和冷卻,使其內部組織發生變化從而獲得預期的性能、組織和結構的工藝過程。退火、正火、淬火、回火是熱處理中的“四把火”,其中的淬火與回火關系密切,常常配合使用,缺一不可。

　　退火是將工件加熱到適當溫度并保溫一定時間,然后進行緩慢冷卻,使其內部組織達到或接近平衡狀態,使前道工序產生的內部應力得以釋放,獲得良好的工藝性能和使用性能為進一步淬火作準備。

　　正火是將工件加熱到適宜的溫度后在空氣中冷卻,正火的效果同退火相似,只是得到的組織更細,常用于改善材料的切削性能,可用于對一些要求不高的零件作為最終熱處理。

　　淬火是將工件加熱保溫后,在水、油或其他無機鹽溶液、有機水溶液等淬冷介質中快速冷卻。

　　回火是將淬火后的工件在高于室溫而低于650℃的適當溫度下進行較長時間的保溫,再進行冷卻,它可降低淬火后工件的脆性。

　　四把火隨著加熱溫度和冷卻方式的不同,又演變出不同的熱處理工藝。為了獲得一定的強度和韌性,把淬火和高溫回火結合起來的工藝,稱為調質。

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　　How to choose surface treatment plating

　　Electroplating is to coat a layer of metal coating on fasteners to change their surface properties, so that they are not easy to oxidize and corrode. Generally, corrosion-resistant metals are used as coating metals.

　　Electroplating can not only enhance the corrosion resistance of fasteners, but also increase the hardness to prevent wear, improve the conductivity, heat resistance, and make the surface more smooth and beautiful.

　　Electroplating is the process of using electrolysis to make a layer of metal film on the surface of metal or other materials. The following is a brief introduction of some common coatings for fasteners.

　　1、 Plating Code:

　　When it comes to the identification of riveting fasteners, it refers to the suffix code of surface treatment, which represents the plating type of the surface.

　　Now we have made a detailed statistics of some common surface treatment methods used in fasteners. The following table:

　　Finish code

　　Zinc clear Zi

　　Zinc blue Zu

　　Zinc yellow ZC

　　Zinc black ZB

　　Nickel flash Ni

　　Chrome flash Cr

　　Tin flash et

　　Natural NL

　　Natural oxide Na

　　Black anosize bl

　　2、 About plating:

　　1. Galvanizing

　　Electroplated zinc plating is the most commonly used coating for fasteners, which has good-looking appearance and low price. It has white zinc, blue zinc, color zinc, black zinc and other colors. Compared with other metal coatings, zinc is relatively cheap and easy to electroplate. However, its corrosion resistance is general. The neutral salt spray test of electro galvanizing is within 72 hours. Of course, special sealer is also used to make the neutral salt spray test more than 200 hours, but the price ratio is 5-8 times higher than that of general galvanizing.

　　2. Electroless nickel plating

　　Nickel plated fasteners are generally used in places with high corrosion resistance and good conductivity. Due to the strong passivation ability of nickel, a very thin passivation film can be rapidly formed on the surface, which can resist the corrosion of atmosphere, alkali and some acids. Electroplated nickel plating has excellent polishing performance, after polishing, its luster can be maintained for a long time, and the high hardness of nickel plating can improve the wear resistance of fasteners.

　　3. Oxidation

　　Oxidation blackening + oiling is a popular coating for industrial fasteners because it is the cheapest and looks good before the oil consumption is exhausted. In the presence of oil, the salt spray test can only reach 3-5 hours. The torque preload consistency of blackened fasteners is also poor. If it needs to be improved, it can be screwed in after applying grease on the internal thread during assembly.

　　4. Chromium plating

　　Chromium coating is generally used as decoration on fasteners. Chromium coating is stable in the atmosphere, not easy to change color and lose luster, high hardness and good wear resistance. Good chrome plated fasteners are as expensive as stainless steel. Only when the strength of stainless steel is not enough, chrome plated fasteners can be used instead. Therefore, they are rarely used in industrial fields with high requirements for corrosion resistance. In order to prevent corrosion, copper and nickel should be plated before chromium plating. Chromium coating can withstand 650 ℃ high temperature, but it has the same hydrogen embrittlement as galvanizing.

　　3、 Plating standard and quality inspection:

　　National standard for fastener surface treatment GB / t5267.1-2002 electroplated coating standard for threaded fasteners, which includes two standards: GB / t5267.1-2002 electroplated coating for fasteners and GB / t5267.2-2002 non electrolytic zinc coating for fasteners. This standard is equivalent to the international standard iso4042-1999 electroplated coating standard for threaded fasteners.

　　The main purpose of fastener surface treatment is to improve the anti-corrosion ability of fasteners, so as to increase the reliability and adaptability of fasteners. The main measurement standard is corrosion resistance, followed by appearance.

　　The quality of electroplated coatings for fasteners is mainly judged from the following aspects:

　　1. Visual inspection

　　The surface of fasteners shall be smooth, glossy and free of missing plating layer, and shall be free of dirt, pores, pinholes, peeling, scorched coating, gray, peeling, skinning, obvious stripes, pockmarks, black plating slag, loose passivation film, cracks, peeling and serious passivation traces.

　　2. Coating thickness

　　The coating thickness of fastener is directly related to its corrosion resistance in the atmosphere, but if it is too thick, it will produce thread interference in the installation process. It is recommended that the general coating thickness is 4-12um.

　　The average thickness of hot-dip galvanizing standard is 54 um (43 um for nominal diameter ≤ 3 / 8), and the minimum thickness is 43 um (37 um for nominal diameter ≤ 3 / 8).

　　3. Coating distribution

　　With different deposition methods, the aggregation mode of the coating on the fastener surface is also different. The plating metal is not evenly deposited on the peripheral edge, and the thick coating is obtained at the corner. In the threaded part of the fastener, the thickest coating is located at the top of the thread, gradually thinned along the side of the thread, and deposited thinnest at the bottom of the thread.

　　On the contrary of hot-dip galvanizing, the deposition tendency of thick coating is the same as that of hot-dip galvanizing, but it is smoother and the thickness is much more uniform on the whole surface.

　　4. Hydrogen embrittlement

　　During the processing and treatment of fasteners, especially in the pickling and alkaline washing before plating and the subsequent plating process, hydrogen is produced in the process of surface absorption of hydrogen atom deposition. When the fastener is tightened, the hydrogen transfers towards the part with the most concentrated stress, which causes the pressure to increase beyond its strength and produces tiny surface fracture. The hydrogen penetrates into the newly formed fracture. This pressure fracture penetration cycle continues until the fastener breaks. It usually occurs within a few hours of the first stress application. To eliminate the threat of hydrogen embrittlement, tighten